Sea bird shelled seed (4)
I believe the answer is:
erne
'sea bird' is the definition.
(I've seen this before)
'shelled seed' is the wordplay.
'shelled' means to look at the middle letters (the outer letters or shell are removed).
'seed' becomes 'kernel' (kernel is a kind of seed).
The centre of 'kernel' is 'ERNE'.
(Other definitions for erne that I've seen before include "A high-flier" , "Fish-eating sea eagle" , "Large bird" , "Light (a flame)" , "one that catches fish?" .)