Enormous insect covering a small distance (7)
I believe the answer is:
mammoth
'enormous' is the definition.
(synonyms)
'insect covering a small distance' is the wordplay.
'insect' becomes 'moth' (moth is a kind of insect).
'covering' is an insertion indicator.
'small distance' becomes 'mm' (abbreviation for millimetre).
'a'+'mm'='amm'
'moth' enclosing 'amm' is 'MAMMOTH'.
(Other definitions for mammoth that I've seen before include "Extinct woolly creature" , "Prehistoric woolly elephant" , "Large extinct beast" , "Extinct species of elephant" , "Enormous" .)