Giant insect covering a couple of metres (7)
I believe the answer is:
'giant' is the definition.
'insect covering a couple of metres' is the wordplay.
'insect' becomes 'moth' (moth is a kind of insect).
'covering' indicates putting letters inside.
'couple of' says to double the letters.
'metres' becomes 'm'.
'm' duplicated is 'mm'.
'moth' enclosing 'amm' is 'MAMMOTH'.
(Other definitions for mammoth that I've seen before include "Brobdingnagian" , "Extinct type of elephant" , "Large extinct elephant" , "Prehistoric woolly elephant" , "Gigantic - extinct animal" .)