Close to rock, gets to swim with a seal (6)
I believe the answer is:
gasket
'seal' is the definition.
(gasket is a kind of seal)
'close to rock gets to swim with a' is the wordplay.
'close to' indicates one should take the final letters (the 'close' to the word).
'to swim with' indicates an anagram.
The last letter of 'rock' is 'k'.
'k'+'gets'='kgets'
'kgets' with its letters anagrammed with 'a' gives 'GASKET'.
(Other definitions for gasket that I've seen before include "packing material" , "Flat sheet used as engine seal" , "It provides a seal" , "Ring for sealing a pipe joint" , "Flat metal seal between tow surfaces as of engine" .)