Component of ammunition (4)
I believe the answer is:
unit
'component' is the definition.
(I know that unit is a type of component)
'of ammunition' is the wordplay.
'of' indicates the answer is hidden within the clue.
'UNIT' is hidden in the letters of 'ammunition'.
(Other definitions for unit that I've seen before include "Single undivided entity" , "Whole - group" , "Piece of furniture" , "Separate part" , "Constituent" .)