Mother, insect's enormous (7)
I believe the answer is:
mammoth
'enormous' is the definition.
(synonyms)
'mother insect's' is the wordplay.
'mother' becomes 'mam' (I've seen this in other clues).
'insect' becomes 'moth' (moth is a kind of insect).
'mam'+'moth'='MAMMOTH'
(Other definitions for mammoth that I've seen before include "Prehistoric beast like hairy elephant" , "Large extinct beast" , "Enormous" , "Extinct shaggy-haired elephant" , "Large, woolly ice-age beast" .)