Huge insect on mother, beginning to move (7)
'huge' is the definition.
'insect on mother beginning to move' is the wordplay.
'insect' becomes 'moth' (moth is a kind of insect).
'on' means one lot of letters go next to another.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'beginning to' says to take the initial letters.
The initial letter of 'move' is 'm'.
'moth' after 'mam' is 'MAMMOTH'.
(Other definitions for mammoth that I've seen before include "Brobdingnagian", "Immense", "Extinct type of elephant", "Enormous", "Huge".)