Seabird unknown in Channel? (5)
I believe the answer is:
gully
'channel?' is the definition.
(gully is a kind of channel)
'seabird unknown' is the wordplay.
'seabird' becomes 'gull' (gull is a kind of seabird).
'unknown' becomes 'y' (algebra).
'gull'+'y'='GULLY'
'in' is the link.
(Other definitions for gully that I've seen before include "Water-worn ravine - fielding position" , "Rainwater channel" , "Water channel; fielding position" , "Fielding position -- narrow channel" , "Cricket fielding position" .)