Copied primate, on the day (4)
I believe the answer is:
aped
'copied' is the definition.
('ape' can be a synonym of 'copy')
'primate on the day' is the wordplay.
'primate' becomes 'ape' (large primate).
'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).
'ape'+'d'='APED'
(Other definitions for aped that I've seen before include "Imitated like primate" , "Imitated like monkey" , "took off" , "was imitative" , "Imitated, mimicked" .)