Club he joined to swim (5)
I believe the answer is:
bathe
'swim' is the definition.
(bathe is a kind of swim)
'club he joined' is the wordplay.
'club' becomes 'bat' (I've seen this before**).
'joined' means one lot of letters go next to another.
'bat'+'he'='BATHE'
'to' acts as a link.
(Other definitions for bathe that I've seen before include "Swim in sea" , "Take a swim; envelop" , "spend time in the water" , "Cleanse the body" , "Suffuse" .)