Swimmer seen in surf is huge (4)
I believe the answer is:
fish
'swimmer' is the definition.
(fish swim)
'in surf is huge' is the wordplay.
'in' indicates a hidden word.
'FISH' can be found hidden inside 'surf is huge'.
'seen' acts as a link.
(Other definitions for fish that I've seen before include "Aquatic animal" , "Cod, dab, eel and ray" , "Ruff" , "Possibly the sole" , "counter in various games" .)