Like a small animal (3)
I believe the answer is:
ass
'animal' is the definition.
(term for a donkey)
'like a small' is the wordplay.
'like' becomes 'as' ().
'a small' becomes 's' (abbreviation - e.g. clothes size).
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "Silly fellow in passionate state" , "twit!" , "Horse relative" , "Blockhead" , "Bottom, temporarily" .)