Part of ammunition (4)
I believe the answer is:
unit
'part' is the definition.
(I know that unit is a type of part)
'of ammunition' is the wordplay.
'of' says the answer is hidden in the clue.
'UNIT' is hidden within 'ammunition'.
(Other definitions for unit that I've seen before include "squad" , "Single element; one" , "Constituent" , "Mexican maize pancake" , "One part of a whole" .)