The animal is equally small (3)
I believe the answer is:
ass
'the animal' is the definition.
(I've seen this before)
'equally small' is the wordplay.
'equally' becomes 'as' ('as' can be a synonym of 'equally').
'small' becomes 's' (abbreviation - e.g. clothes size).
'as'+'s'='ASS'
'is' acts as a link.
(Other definitions for ass that I've seen before include "transporter" , "Clot" , "Transport for a wise man - or a fool" , "Stupid person or American bum" , "Animal related to horses" .)