Young animal's mother back inside pound (4)
I believe the answer is:
lamb
'young animal's' is the definition.
(young sheep)
'mother back inside pound' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'back' shows that the letters should be reversed in order.
'inside' indicates putting letters inside.
'pound' becomes 'lb' (abbreviation for the unit of weight).
'ma' reversed gives 'am'.
'am' inserted within 'lb' is 'LAMB'.
(Other definitions for lamb that I've seen before include "Noted essayist - symbol of innocence" , "Tender sheep meat" , "Charles ---, essayist; meek animal" , "Writer" , "Youngster" .)