Poser troubled unknown raptor (6)
I believe the answer is:
osprey
'raptor' is the definition.
(osprey is a kind of raptor)
'poser troubled unknown' is the wordplay.
'troubled' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'poser' is an anagram of 'ospre'.
'ospre'+'y'='OSPREY'
(Other definitions for osprey that I've seen before include "Fish eagle" , "Fish-eating raptor" , "predatory bird" , "I grab fish" , "Large bird preying on fish in inland waters" .)