Sea rangers regularly observed seabirds (5)
I believe the answer is:
ernes
'seabirds' is the definition.
(erne can mean a sea eagle)
'sea rangers regularly observed' is the wordplay.
'regularly observed' means one should take alternating letters (letters selected at regular intervals).
'sea'+'rangers'='searangers'
The alternating letters of 'searangers' are 'ERNES'.
(Other definitions for ernes that I've seen before include "sea eagles" , "Sea-eagles" , "They fly across the sea" , "Irish lough's" , "Certain birds" .)