Seven of the eight nine-letter answers contain three (and only three) consecutive letters from the word SEPTEMBER, in the same position. The clues for these omit the three relevant letters from the wordplay.
Across:
1.
Pure sustained attack allowed to ferment (13)
10.
Port island very backwards (5)
11.
Snaky liar in scandal (9)
12.
Conceal Detective Inspector with less sophistication (9)
13.
Saying nothing after French word a bit tawdry (5)
14.
Building conversation with Belarus at regular intervals (7)
16.
Cure for lack of guitar playing? (7)
18.
Bone method fit for relating to absorption (7)
20.
Reactant pair found in chicken compound in liver (7)
22.
Software license features reminder (5)
24.
Rash limit (pico-cycles) (9)
26.
Pairs of investigators scrutinise rich writer (9)
27.
Ground sweetheart down for Yankee (5)
28.
Pernicious ban on gamer and rubbish dealer (3-3-4,3)
Down:
2.
Woman has facial hair in new area? Ace (7)
3.
Reduction beat an administration (9)
4.
Invested in wealth, robbing pound (5)
5.
Yet another uniform soldiers take home initially (9)
6.
Fool with maiden replacing shirt, so to speak? (5)
7.
Wet magazine returned to sender? (7)
8.
Energy supplier circular - do not open! (9,4)
9.
Salary mostly with Greek character - can I do harm without talking? (13)