Holding fire at first, annoy NCOs in fighting force (5,5)
I believe the answer is:
rifle corps
'fighting force' is the definition.
(army regiment)
'holding fire at first annoy ncos' is the wordplay.
'holding' means one lot of letters goes inside another.
'at first' says to take the initial letters.
'annoy' becomes 'rile' (synonyms).
'ncos' becomes 'corps' (non-commissioned officer corps).
The initial letter of 'fire' is 'f'.
'f' put inside 'rile' is 'rifle'.
'rifle'+'corps'='RIFLE-CORPS'
'in' acts as a link.
(Other definitions for rifle corps that I've seen before include "Armed troops" , "Soldiers" .)