Am in pound for an animal (4)
I believe the answer is:
lamb
'for an animal' is the definition.
(lamb is a kind of animal)
'am in pound' is the wordplay.
'in' means one lot of letters goes inside another.
'pound' becomes 'lb' (abbreviation for the unit of weight).
'am' going within 'lb' is 'LAMB'.
(Other definitions for lamb that I've seen before include "Young farm animal" , "Agnus Dei - . . . . of God" , "Youngster" , "Sweet and innocent child" , "English essayist" .)