Channel goes under stone support (5)
I believe the answer is:
strut
'support' is the definition.
(strut is a kind of support)
'channel goes under stone' is the wordplay.
'channel' becomes 'rut' (I've seen this before).
'goes under' is a charade indicator (letters next to each other).
'stone' becomes 'st'.
'rut' after 'st' is 'STRUT'.
(Other definitions for strut that I've seen before include "Walk with a lofty proud gait" , "Swagger; bracing bar" , "Walk proudly like metal support" , "Display arrogance" , "Walk proudly like bantam-cock" .)