Swimming, perturb ocean swelling (12)
I believe the answer is:
protuberance
'swelling' is the definition.
(I know that swelling can be written as protuberance)
'swimming perturb ocean' is the wordplay.
'swimming' is an anagram indicator (letters swim around into a new order).
'perturb'+'ocean'='perturbocean'
'perturbocean' anagrammed gives 'PROTUBERANCE'.
(Other definitions for protuberance that I've seen before include "Swelling" , "Bit jutting out" , "Outward bulge - not cupbearer (anag)" , "Something that sticks out" , "Something sticking out" .)