Stag partygoer's ending in Panama, perhaps (4)
I believe the answer is:
hart
'stag' is the definition.
('hart' can be a synonym of 'stag')
'partygoer's ending in panama perhaps' is the wordplay.
'ending' suggests the final letters.
'in' means one lot of letters goes inside another.
'panama perhaps' becomes 'hat' (panama is a kind of hat).
The last letter of 'partygoer' is 'r'.
'r' inserted inside 'hat' is 'HART'.
(Other definitions for hart that I've seen before include "Mature male deer" , "Deer; Lorenz --, lyricist" , "Richard Rodgers and Lorenz . . . . were famous musical partnership" , "Ruminative type" , "Male deer; Rodgers collaborator" .)