Massive flier covers a short distance (7)
I believe the answer is:
mammoth
'massive' is the definition.
(thesaurus)
'flier covers a short distance' is the wordplay.
'flier' becomes 'moth' (something which flies).
'covers' means one lot of letters goes inside another.
'short distance' becomes 'mm' (abbreviation for millimetre).
'a'+'mm'='amm'
'moth' enclosing 'amm' is 'MAMMOTH'.
(Other definitions for mammoth that I've seen before include "Enormous" , "Extinct species of elephant" , "Extinct shaggy-haired elephant" , "Gigantic - extinct animal" , "Prehistoric woolly elephant" .)