Opponent splitting bright beam (10)
I believe the answer is:
Here is my best explanation:
'beam' is the definition.
(cantilever is a kind of beam)
'opponent splitting bright' is the subsidiary indication.
'opponent' becomes 'anti' (I've seen this before).
'splitting' is an insertion indicator.
'bright' becomes 'clever' (I've seen this before**).
'anti' placed within 'clever' is 'cantilever'.
(Other definitions for cantilever that I've seen before include "Projecting beam fixed at one end", "Beam supported at only one end" and "Beam fixed at one end".)