Swimmer regularly at sea (Torbay) (5)
I believe the answer is:
'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.).
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" .)