On this day, man cooked (5)
I believe the answer is:
fried
'cooked' is the definition.
('fried' can be similar in meaning to 'cooked')
'on this day man' is the wordplay.
'on' says to put letters next to each other.
'this day' becomes 'fri' (**).
'man' becomes 'ed'.
'fri' next to 'ed' is 'FRIED'.
(Other definitions for fried that I've seen before include "Cooked in oil" , "Cooked in oil/fat" , "Cooked on a hot surface" , "Such as cooking chips" , "Cooked in fat" .)