Beam is against assuming cunning (10)
I believe the answer is:
cantilever
'beam' is the definition.
(cantilever is a kind of beam)
'against assuming cunning' is the wordplay.
'against' becomes 'anti' (I've seen this before).
'assuming' means one lot of letters goes inside another (assume can mean to take on).
'cunning' becomes 'clever' (synonyms).
'anti' put into 'clever' is 'CANTILEVER'.
'is' acts as a link.
(Other definitions for cantilever that I've seen before include "support at one end" , "Beam supported at only one end" , "Projecting mass" , "bridge design" , "Projecting beam fixed at one end" .)