Against cutting sharp supporting beam (10)
I believe the answer is:
cantilever
'supporting beam' is the definition.
(type of beam)
'against cutting sharp' is the wordplay.
'against' becomes 'anti' (I've seen this in another clue).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'sharp' becomes 'clever' (I've seen this before).
'anti' inserted inside 'clever' is 'CANTILEVER'.
(Other definitions for cantilever that I've seen before include "Beam supported at only one end" , "Projecting mass" , "support at one end" , "Projecting beam fixed at one end" , "bridge design" .)