Small animal's ration (5)
I believe the answer is:
share
'ration' is the definition.
(a ration is a share of a limited resource)
'small animal's' is the wordplay.
I cannot really understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
'animal' could be 'hare' (I have seen 'Swift animal ' mean 'hare' so perhaps 'animal' could also mean 'hare') and 'hare' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for share that I've seen before include "Portion, maybe of capital" , "Contribution; portion" , "Helping" , "hand pieces out" , "Allotted portion" .)