Fixed allowance of one sailor back working (6)
I believe the answer is:
ration
'fixed allowance' is the definition.
(a fixed quota given out to someone)
'one sailor back working' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sailor' becomes 'tar' (slang term for a sailor).
'back' shows that the letters should be reversed in order.
'working' becomes 'on' (on can mean working or activated).
'i'+'tar'='itar'
'itar' backwards is 'rati'.
'rati'+'on'='RATION'
'of' acts as a link.
(Other definitions for ration that I've seen before include "Food apportionment" , "Parcel out scarce food, say" , "Share or allotment, of food say" , "A fixed allowance in times of scarcity" , "Limit quantity as of food during scarcity" .)