Fixed allowance under old address (7)
I believe the answer is:
oration
'address' is the definition.
(oration is a kind of address)
'fixed allowance under old' is the wordplay.
'fixed allowance' becomes 'ration' (I've seen this before).
'under' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ration' after 'o' is 'ORATION'.
(Other definitions for oration that I've seen before include "A formal or ceremonial speech" , "Platform speech" , "eulogy" , "Ontario (anag)" , "Power of speech" .)