Mother insect is huge
I believe the answer is:
mammoth
'huge' is the definition.
(synonyms)
'mother insect' 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'
'is' acts as a link.
(Other definitions for mammoth that I've seen before include "Extinct hairy elephant" , "Large 15 animal" , "Immense" , "Prehistoric beast like hairy elephant" , "Large extinct beast" .)