One piece of ammunition (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'of ammunition' is the wordplay.
'of' says the answer is hidden in the clue.
'UNIT' is hidden within 'ammunition'.
'piece' acts as a link.
(Other definitions for unit that I've seen before include "Single undivided entity" , "Piece of furniture" , "Self-contained section" , "Single piece - kitchen cabinet" , "Individual item" .)