Punt in which to practise going on river (5)
I believe the answer is:
wager
'punt' is the definition.
(thesaurus)
'to practise going on river' is the wordplay.
'to practise' becomes 'wage' (I've seen this before**).
'going on' means one lot of letters go next to another.
'river' becomes 'r'.
'wage'+'r'='WAGER'
'in which' acts as a link.
(Other definitions for wager that I've seen before include "stake required" , "A bet or stake" , "better to put this on?" , "One making pledge" , "Bet, gamble" .)