Explanations

The theme of the puzzle was the 'Movember' charity event (the "growing movement"!). Therefore, the asterisked clues all referred to the first name/surname of famous mustache sporters, with the other half of their name to be entered in the grid. 22a and 23a together (IFYOUMUST + ACHES) revealed the hidden MUSTACHE(S).

Sincere apologies for the error in 24d.

Across

1. D (down) contained in SADEN (a dense endless spiral) => SADDEN (&lit)
2. BRUT (Dry in drinking terms) moving R (right) forward => BURT (REYNOLDS)
3. LET (tolerated) containing VI (6) + D (down) after A (one) + TE (note) => LEVITATED (raised)
4. Doubt minus (sacrificing) D (dead) and B (British) + ER (Queen) => OUTER (beyond)
5. A (one) + IS + LET (allowed) minus T (start Trailing out) => AISLE (row)
6. LOW (flat) + COD (fish) containing ME (I walked into) + Y (yes) => LOW COMEDY (it's slapstick)
7. Contained in (held) lOVER RUNning => OVERRUN (encircle)
8. Anagram (getting drunk) spotted minus T (taking time out) => DESPOT (21d = Hitler)
9. Anagram (damage) causin (predominantly causing) => ACINUS (a bit of fruit)
10. AEL (ale contest) contained in IS + R (run) + I (Independent) => ISRAELI (national)
11. IFY (FYI dressed) + MUS (all =sum backwards) contained in OUT (with dated trimming) => IF YOU MUST (oh go on then)
12. AS containing CHE (revolutionary leader is captured) => ACHES (agonises)
13. Contained in (bashing into) guerilLA THIeves => LATHI (Indian truncheon)
14. Anagram (shakes) cinema + mogul minus AU (without gold) => COMMINGLE (blend)
15. RED (flaming) + O (Oscar) + LENT (temporarily given) => REDOLENT (something memorable)
16. PEN (writer) + initials (starts) Nervous Output Now => PENNON (flag)

Down

1. I (slice of Imagination + LAD (boy) backward (brought up) => DALI (SALVADOR)
2. SAY (speak) containing (about) MM (small metric measurement = millimetre) => SAMMY (DAVIS)
3. Anagram (fiendishly) repeat + lie + runner => ENTREPRENEURIAL (selling spirit)
4. END (the last bit) + OWED (due) => ENDOWED (given)
5. Anagram (desperately) naive + cover + notes => NEOCONSERVATIVE (of old standards)
6. LIT (on) + HE (man) + SOME (bit) => LITHESOME (elastic)
7. STS (streets) containing RAY (singer Mr Charles) => STRAYS (runaways)
8. J (jack) + OF containing (captured) S (spades) + E (east) => JOSEF (STALIN)
9. N (name) + retyped + C (Charlie) anagram (foxtrot) => ENCRYPTED (in code)
10. AL (nearly all) + B (black) + E (England) + RT (RioT on the fringes) => ALBERT (EINSTEIN)
11. SOON (presently) containing (gathering) UP (high) + C (carbon) => SOUPCON (trace)
12. IN (between) + TIM (director Burton) + E (Elfman, primarily) => IN TIME (synchronised)
13. A (second in hAnd) + DOLF (fold, regretfully) => ADOLF (HITLER)
14. HULL (keel) with L replaced by K (**Kismet debut replaced by learner**) => HULK (HOGAN)

**Note that this was an unspotted error in the clue.