Attack keep — insurgents held back guards (5)
I believe the answer is:
snipe
'attack' is the definition.
(thesaurus)
'keep insurgents held back guards' is the wordplay.
'guards' says the answer is hidden in the clue.
'held back' is a reversal indicator.
'epins' can be found hidden inside 'keep insurgents'.
'epins' reversed gives 'SNIPE'.
(Other definitions for snipe that I've seen before include "The bird to shoot from cover" , "Pines like wading bird" , "Curlew or sandpiper" , "Verbal criticism" , "Criticise" .)