Forbidding party game to be taken up (4)
I believe the answer is:
dour
'forbidding' is the definition.
(I know that forbidding can be written as dour)
'party game to be taken up' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'game' becomes 'RU' (rugby union).
'to be taken up' shows that the letters should be reversed in order.
'ru' in reverse letter order is 'ur'.
'do'+'ur'='DOUR'
(Other definitions for dour that I've seen before include "Severe, stern, gloomy" , "Stern and forbidding" , "Erratic" , "Lugubrious, grim" , "Grim, morose" .)