Bird, unknown, in valley (5)
I believe the answer is:
gully
'valley' is the definition.
(thesaurus)
'bird unknown' is the wordplay.
'bird' becomes 'gull' (gull is a kind of bird).
'unknown' becomes 'y' (algebra).
'gull'+'y'='GULLY'
'in' is the link.
(Other definitions for gully that I've seen before include "Cricket position; ravine" , "In field, position" , "Deep ditch" , "Fielding position -- narrow channel" , "Ravine cut by running water" .)