A reply that's silly for an actor (6)
I believe the answer is:
player
'for an actor' is the definition.
('player' can be a synonym of 'actor')
'a reply that's silly' is the wordplay.
'that's silly' indicates an anagram.
'a'+'reply'='areply'
'areply' anagrammed gives 'PLAYER'.
(Other definitions for player that I've seen before include "Gamer or sportsman" , "Sportsperson; actor" , "powerful participant" , "Sports team member" , "Participant in competitive game" .)