Despicable person, one on allotment (6)
I believe the answer is:
ration
'allotment' is the definition.
(I've seen this in other clues)
'despicable person one on' is the wordplay.
'despicable person' becomes 'rat' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'rat'+'i'+'on'='RATION'
(Other definitions for ration that I've seen before include "Quota" , "Dole out in measured quantities, as in time of shortage" , "Dole out portion due to shortage" , "Helping" , "Portion, share" .)