Huge swimmer (5)
I believe the answer is:
whale
'huge swimmer' is the definition.
'whale' can be an answer for 'swimmer' (thesaurus). I also know that 'huge' relates to this answer.
This is the entire clue.
(Other definitions for whale that I've seen before include "Large marine mammal" , "Patch of oil, on water" , "Aquatic mammal" , "It breathes through a blowhole" , "Maybe humpback" .)