Revolutionary seems to lose heart in game (5)
I believe the answer is:
chess
'game' is the definition.
(chess game is a kind of game)
'revolutionary seems to lose heart' is the wordplay.
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'to lose heart' suggests removing the centre (I've seen 'losing heart' mean this).
'seems' with its middle taken out is 'ss'.
'che'+'ss'='CHESS'
'in' is the link.
(Other definitions for chess that I've seen before include "Benny Andersson and Bjorn Ulvaeus musical" , "Pure-skill board game" , "Ancient war game whose name may have derived for the Persian for 'king'" , "Game with kings and queens" , "that has over thirty men on board" .)