Jack used to play this sport (5)
I believe the answer is:
bowls
'jack used to play this sport' is the definition.
'bowls' can be an answer for 'sport' (thesaurus). I'm not sure about the remainder of the definition.
'play this sport' is the wordplay.
'play' becomes 'bow' (bowing is a kind of playing).
'this sport' becomes 'ls' (I can't justify this - if you can you should believe this answer much more).
'bow'+'ls'='BOWLS'
Can you help me to learn more?
(Other definitions for bowls that I've seen before include "Kitchen vessels" , "Kitchen pots for storing" , "woods" , "Throws or rolls a ball" , "Game; vessels" .)