Fix soldiers on the day (4)
I believe the answer is:
mend
'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' says to put letters next to each other (in a down clue, letters appear on others).
'the day' becomes 'd' (abbreviation for day).
'men'+'d'='MEND'
(Other definitions for mend that I've seen before include "Put right" , "Heal" , "'Restore, fix (4)'" , "Fix, repair" , "Make improvements" .)