Firm support brings disappointment (7)
I believe the answer is:
setback
'disappointment' is the definition.
(I've seen this before)
'firm support' is the wordplay.
'firm' becomes 'set' (both can mean fixed).
'support' becomes 'back' (to back is to support or endorse).
'set'+'back'='SETBACK'
'brings' acts as a link.
(Other definitions for setback that I've seen before include "Hindrance to progress" , "Problem" , "disappointment" , "Reversal of fortune" , "Reverse" .)