Actor on series gets hamper (9)
I believe the answer is:
hamstring
'hamper' is the definition.
(hamstring can mean to hamper or obstruct)
'actor on series' is the wordplay.
'actor' becomes 'ham' (ham actor is a kind of actor).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'series' becomes 'string' (sequence of similar items or events).
'ham'+'string'='HAMSTRING'
'gets' acts as a link.
(Other definitions for hamstring that I've seen before include "Impede" , "Tendon at back of knee" , "Thwart, severely restrict" , "Tendons at back of the knee" , "hamper" .)