Poser trounced in game (6)
I believe the answer is:
enigma
'poser' is the definition.
(I've seen this in another clue)
'trounced in game' is the wordplay.
'trounced' indicates an anagram.
'in'+'game'='ingame'
'ingame' anagrammed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Riddle - German coding machine" , "Mysteriously puzzling person or thing" , "WW two Ger. code device; puzzle" , "cryptic puzzle!" , "'Puzzle, riddle (6)'" .)