Chimney bulge beginning to open -- a pain (7)
I believe the answer is:
lumbago
'a pain' is the definition.
(lumbago is a kind of pain)
'chimney bulge beginning to open' is the wordplay.
'chimney' becomes 'lum' (I've seen this in other clues).
'bulge' becomes 'bag' ('bag' can be a synonym of 'bulge').
'beginning to' suggests taking the first letters.
The first letter of 'open' is 'o'.
'lum'+'bag'+'o'='LUMBAGO'
(Other definitions for lumbago that I've seen before include "Form of backache" , "Back affliction" , "Pain -- a bum log (anag)" , "Back complaint" , "Back problem" .)