Marsh bird - part sea bird (7)
I believe the answer is:
bittern
'marsh bird' is the definition.
(I've seen this before)
'part sea bird' is the wordplay.
'part' becomes 'bit' (bit is a kind of part).
'sea bird' becomes 'tern' (tern is a kind of sea bird).
'bit'+'tern'='BITTERN'
(Other definitions for bittern that I've seen before include "Wading bird" , "Bird with a booming cry" , "Source of boom" , "Bird that booms" , "flier" .)