Sport in which Jack plays important role? (5)
I believe the answer is:
bowls
This clue might be a double definition.
'sport in which jack' is the first definition.
'bowls' can be an answer for 'sport' (thesaurus). I'm not sure about the remainder of the definition.
'plays important role?' is the second definition.
'bowls' can be an answer for 'plays' (bowling is a kind of playing). I'm not certain of the 'important role?' bit.
Can you help me to learn more?
(Other definitions for bowls that I've seen before include "Game; vessels" , "Containers for fruit" , "woods" , "Throws or rolls a ball" , "They're rolled towards jack" .)