Hell for quiet supporter of mine! (7)
I believe the answer is:
pitprop
'supporter of mine' is the definition.
(structural support in a mine)
'hell for quiet' is the wordplay.
'hell' becomes 'pit' ('the pit' can mean hell).
'for' becomes 'pro' (as in being 'pro' something).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'pit'+'pro'+'p'='PITPROP'
(Other definitions for pitprop that I've seen before include "Mine-working support" , "Mine support" , "supporter of mine" .)