Fix soldiers on the day (4)
I believe the answer is:
'fix' is the definition.
(I know that fix can be written as mend)
'soldiers on the day' is the wordplay.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'on' is a charade indicator (letters next to each other).
'the day' becomes 'd' (abbreviation for day).
(Other definitions for mend that I've seen before include "Put right" , "Heal" , "'Restore, fix (4)'" , "Fix, repair" , "Make improvements" .)