Swimmer regularly at sea (Torbay) (5)
I believe the answer is:
tetra
'swimmer' is the definition.
(tetra is a kind of fish)
'regularly at sea torbay' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'at'+'sea'+'torbay'='atseatorbay'
The alternate letters of 'atseatorbay' are 'TETRA'.
(Other definitions for tetra that I've seen before include "Small tropical fish" , "Bright tropical fish" , "Wrongful act (Law)" , "rarely seen flower" , "This indicates a group of four" .)