Whiskey in can, a double? (4)
I believe the answer is:
twin
'a double?' is the definition.
(twin can mean someone's double)
'whiskey in can' is the wordplay.
'whiskey' becomes 'W' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'can' becomes 'tin' (both are metal containers).
'w' placed into 'tin' is 'TWIN'.
(Other definitions for twin that I've seen before include "This set is matching jumper and cardigan" , "One of two conceived together" , "Link (two towns)" , "One born with another" , "Family member" .)