Reason head of garrison gets ammunition (7)
I believe the answer is:
grounds
'reason' is the definition.
(grounds or justification for a particular action)
'head of garrison gets ammunition' is the wordplay.
'head of' says to take the initial letters.
'gets' is a charade indicator (letters next to each other).
'ammunition' becomes 'rounds' (I've seen this before).
The first letter of 'garrison' is 'g'.
'g'+'rounds'='GROUNDS'
(Other definitions for grounds that I've seen before include "enclosure around house" , "Basis for belief" , "they make coffee" , "Refuses authority to fly" , "Area of enclosed land around a building" .)