Opponent splitting bright beam (10)
I believe the answer is:
cantilever
'beam' is the definition.
(cantilever is a kind of beam)
'opponent splitting bright' is the wordplay.
'opponent' becomes 'anti' (I've seen this before).
'splitting' indicates putting letters inside.
'bright' becomes 'clever' (I've seen this before**).
'anti' inserted within 'clever' is 'CANTILEVER'.
(Other definitions for cantilever that I've seen before include "Projecting mass" , "Beam supported at only one end" , "support at one end" , "Projecting beam fixed at one end" , "bridge design" .)