Poor actor on series is handicap (9)
I believe the answer is:
hamstring
'is handicap' is the definition.
(thesaurus)
'poor actor on series' is the wordplay.
'poor actor' becomes 'ham' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'series' becomes 'string' (sequence of similar items or events).
'ham'+'string'='HAMSTRING'
(Other definitions for hamstring that I've seen before include "Tendons at back of the knee" , "Cramp the style of" , "Impede" , "Tendon behind the knee" , "Thwart, severely restrict" .)