Fix troops on the day (4)
I believe the answer is:
mend
'fix' is the definition.
('mend' can be a synonym of 'fix')
'troops on the day' is the wordplay.
'troops' becomes 'men' (I've seen this in other clues).
'on' is a charade indicator (letters next to each other).
'the day' becomes 'd'.
'men'+'d'='MEND'
(Other definitions for mend that I've seen before include "Return to health; repair" , "Fix, repair" , "'Restore, fix (4)'" , "Put right" , "Heal" .)