Bird seen regularly in the briny (4)
I believe the answer is:
tern
'bird' is the definition.
(tern is a kind of bird)
'regularly in the briny' is the wordplay.
'regularly in' means one should take alternating letters.
'the'+'briny'='thebriny'
The alternating letters of 'thebriny' are 'TERN'.
'seen' is the link.
(Other definitions for tern that I've seen before include "Seabird, sounds like become sour" , "Seagull" , "Rent out the bird" , "Small slender gull" , "Seabird related to the gull" .)