Bird very upset by game (6)
I believe the answer is:
osprey
'bird' is the definition.
(osprey is a kind of bird)
'very upset by game' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'upset' is a reversal indicator.
'by game' becomes 'prey' (both can mean animals which are hunted. I am not sure about the 'by' bit.).
'so' in reverse letter order is 'os'.
'os'+'prey'='OSPREY'
(Other definitions for osprey that I've seen before include "Fish eagle" , "predatory bird" , "Flyer" , "Large bird preying on fish in inland waters" , "Large hawk, feeds on fish" .)