Sea eagle's gross, we're told (4)
I believe the answer is:
erne
'sea eagle's' is the definition.
(erne is a kind of sea eagle)
'gross we're told' is the wordplay.
'gross' becomes 'earn' (I've seen this in another clue).
'we're told' shows a homophone (sound like).
'earn' is a homophone of 'ERNE'.
(Other definitions for erne that I've seen before include "flyer in the main" , "Irish lough" , "Seabird" , "European sea eagle" , "Great lake in Fermanagh with lots of islands" .)