Bird in stream hiding head (4)
I believe the answer is:
rook
'bird' is the definition.
(rook is a kind of bird)
'stream hiding head' is the wordplay.
'stream' becomes 'brook' (I've seen this before).
'hiding head' means to remove the first letter.
'brook' with its initial letter removed is 'ROOK'.
'in' is the link.
(Other definitions for rook that I've seen before include "Chess piece or crow" , "Bird; playing piece" , "Steal - castle" , "man in marginalised position, initially" , "Swindle; castle" .)