Although fifty, entering game (6)
I believe the answer is:
whilst
'although' is the definition.
(I've seen this before)
'fifty entering game' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'entering' means one lot of letters goes inside another.
'game' becomes 'whist' (whist is a card game).
'l' inserted within 'whist' is 'WHILST'.
(Other definitions for whilst that I've seen before include "At the same time, of old" , "Whereas" , "As long as; although" .)