Fix chaps on the day (4)
I believe the answer is:
mend
'fix' is the definition.
(I know that fix can be written as mend)
'chaps on the day' is the wordplay.
'chaps' becomes 'men' (chap means a man).
'on' is a charade indicator (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 "Heal" , "Make improvements" , "Fix, repair" , "Put right" , "'Restore, fix (4)'" .)