Old club whiskey's source, that is (6)
I believe the answer is:
mashie
'old club' is the definition.
(obsolete golf club)
'whiskey's source that is' is the wordplay.
'whiskey's source' becomes 'mash' (whiskey is made from grain mash).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'mash'+'ie'='MASHIE'
(Other definitions for mashie that I've seen before include "Golf club for lofting" , "Old-fashioned golf club" , "Old golf club" , "One of 9 [CLUBS]" , "He aims (anag) - golfer's five iron, formerly" .)