Severely restrict one acting badly on line (9)
I believe the answer is:
hamstring
'severely restrict' is the definition.
(I've seen this before)
'one acting badly on line' is the wordplay.
'one acting badly' becomes 'ham' (to ham is to overact).
'on' says to put letters next to each other.
'line' becomes 'string' (string line is a kind of line).
'ham'+'string'='HAMSTRING'
(Other definitions for hamstring that I've seen before include "Tendon behind the knee" , "Tendons at back of the knee" , "Cramp the style of" , "hamper" , "Impede" .)