Something of a thug, or, I'll add, an ape (7)
I believe the answer is:
'an ape' is the definition.
(gorilla is a kind of ape)
'something of a thug or i'll add' is the wordplay.
'something of' indicates a hidden word (I've seen 'something' mean this (I've seen this in other clues)).
'GORILLA' is hidden in the letters of 'thug or ill add'.
(Other definitions for gorilla that I've seen before include "brutal man" , "huge beast" , "thug" , "Largest anthropoid ape" , "Powerful great ape" .)