Informer, one on fixed allowance (6)
I believe the answer is:
ration
'fixed allowance' is the definition.
(a fixed quota given out to someone)
'informer one on' is the wordplay.
'informer' becomes 'rat' (synonyms).
'one' becomes 'i' (Roman numeral).
'rat'+'i'+'on'='RATION'
(Other definitions for ration that I've seen before include "Parcel out scarce food, say" , "A measured portion in times of scarcity" , "Restrict the consumption of something" , "Fixed restricted allowance" , "Apportion" .)