The puzzle concerns the 10a 21a, discovered by 25a and 4d in 11a (all clued by wordplay only). To assist with the theme, 4 further answers have each had 5 pertinent letters removed before entry. Clues for these 4 consist of a full definition, but with wordplay for the modified version only.
Across:
6.
New loco rock (loud metal) sounding (5)
7.
That hurt grabbing edges of vase rim more than necessary (8)
10.
Frost to complain, say (7)
11.
Kilimanjaro, finally, after hill not conquered (7)
12.
Run to Battle clothed in fresh air? Tourists will visit! (7)
13.
Last bit of problem covered by nurse (4,3)
14.
Fixes hard knot inside quarters? (5,6)
19.
Raise track speed back with energy (7)
21.
Second upper left between hips? (7)
23.
Leading inspector following suspicious aims of bad guys (7)
25.
Can get into Wham (7)
26.
Ring to learn IT programming in Java? (8)
27.
Confined ecstasy inside master spy (5)
Down:
1.
He inscribes always with point on silver mostly? (8)
2.
Sad, so comes back in again (6)
3.
Disastrously lose after top card inserted in deck (10)
4.
Live on street (4)
5.
Optics, for instance? That is replaced by old candlestick (6)
6.
Join company on board (6)
8.
Fighting shop to get one beer discount? (7)
9.
Down low, down at bottom of valley (5)
13.
Returning stray bag to dance round in Court? (6,4)
15.
Stick around because ... (7)
16.
... sailors suppressing anger (more than one cross) (8)
17.
Wembley gardener's working day marks latest commemoration (5)
18.
Weave around great height to get horse on board (6)