Massive insect underneath mother (7)
I believe the answer is:
mammoth
'massive' is the definition.
(thesaurus)
'insect underneath mother' is the wordplay.
'insect' becomes 'moth' (moth is a kind of insect).
'underneath' says to put letters next to each other (in a down clue, some letters go under others).
'mother' becomes 'mam' (I've seen this in other clues).
'moth' after 'mam' is 'MAMMOTH'.
(Other definitions for mammoth that I've seen before include "Large extinct beast" , "Extinct woolly creature" , "Prehistoric beast like hairy elephant" , "Large extinct elephant" , "Huge" .)