Marine bird occupying higher nests (4)
I believe the answer is:
erne
'marine bird' is the definition.
(I've seen this in another clue)
'occupying higher nests' is the wordplay.
'occupying' says the answer is hidden in the clue (letters for the answer occupy part of the clue).
'ERNE' is hidden in the letters of 'higher nests'.
(Other definitions for erne that I've seen before include "Island-studded lake in Fermanagh" , "flyer in the main" , "Large bird" , "Irish river, loughs; sea bird" , "one that catches fish?" .)