One sailor turned up on allotment (6)
I believe the answer is:
ration
'allotment' is the definition.
(I've seen this in other clues)
'one sailor turned up on' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sailor' becomes 'tar' (tar is a kind of sailor).
'turned up' shows that the letters should be reversed in order.
'i'+'tar'='itar'
'itar' written backwards gives 'rati'.
'rati'+'on'='RATION'
(Other definitions for ration that I've seen before include "Helping" , "A measured portion in times of scarcity" , "Limit, share" , "Parcel out scarce food, say" , "Restrict the supply of" .)