Spider natural at swimming (9)
I believe the answer is:
tarantula
'spider' is the definition.
(I know that tarantula is a type of spider)
'natural at swimming' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'natural'+'at'='naturalat'
'naturalat' anagrammed gives 'TARANTULA'.
(Other definitions for tarantula that I've seen before include "Leggy creature" , "web developer" , "web architect" , "Large venomous spider" , "A big hairy spider" .)