Beam, having ability to restrain opponent (10)
I believe the answer is:
cantilever
'beam' is the definition.
(I know that cantilever is a type of beam)
'having ability to restrain opponent' is the wordplay.
'having ability' becomes 'clever' (clever can mean able or competent).
'to restrain' means one lot of letters goes inside another.
'opponent' becomes 'anti' (someone opposed to something).
'clever' placed around 'anti' is 'CANTILEVER'.
(Other definitions for cantilever that I've seen before include "Beam supported at only one end" , "bridge design" , "Projecting mass" , "Projecting beam fixed at one end" , "support at one end" .)