A fixed allowance following the fashion for self-restraint (10)
I believe the answer is:
moderation
'self-restraint' is the definition.
(I've seen this before)
'a fixed allowance following the fashion' is the wordplay.
'a fixed allowance' becomes 'ration' (a fixed quota given out to someone).
'following' is a charade indicator (letters next to each other).
'the fashion' becomes 'mode' ('mode' can be a synonym of 'fashion').
'ration' put after 'mode' is 'MODERATION'.
'for' is the link.
(Other definitions for moderation that I've seen before include "restraint" , "Avoidance of extremes" , "avoids extremes" , "self-restraint" , "self-control" .)