Prohibit support for musician (8)
I believe the answer is:
banjoist
'for musician' is the definition.
(I've seen this before)
'prohibit support' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'support' becomes 'joist' (joist is a kind of support).
'ban'+'joist'='BANJOIST'
(Other definitions for banjoist that I've seen before include "plucky player" , "Player of stringed instrument" , "plucky performer" , "Musician" .)