Copy first of adult games (3)
I believe the answer is:
ape
'copy' is the definition.
(ape can mean to copy or imitate)
'first of adult games' is the wordplay.
'first of adult' becomes 'a' (1st letter of 'adult').
'games' becomes 'PE' (abbreviation for Physical Education).
'a'+'pe'='APE'
(Other definitions for ape that I've seen before include "drill" , "Parrot" , "Large primate" , "Primate resembling a human" , "Simian - copy" .)