One nursing tot got a drunk to be an animal (5,4)
I believe the answer is:
nanny goat
'animal' is the definition.
(I have seen 'Female animal' mean 'nanny goat' so perhaps 'animal' could also mean 'nanny goat')
'one nursing tot got a drunk' is the wordplay.
'one nursing tot' becomes 'nanny' (a nanny looks after children).
'drunk' means one lot of letters goes inside another (inserted letters are drunk in).
'got' enclosing 'a' is 'goat'.
'nanny'+'goat'='NANNY-GOAT'
'to be an' is the link.
(Other definitions for nanny goat that I've seen before include "Billy's partner?" , "Butter" , "milk producer" , "Female farm animal" .)