Actor Astin (4)
I believe the answer is:
sean
'actor astin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sean that I've seen before include "- - Connery (007)" , "guy at Number 10 [IRISHMAN]" , "Penn, Bean or O'Casey" , "Irish form of John" .)