Tango is bubbly and cold? One's doubtful (8)
I believe the answer is:
agnostic
'one's doubtful' is the definition.
(someone unsure about religious beliefs)
'tango is bubbly and cold?' is the wordplay.
'bubbly' indicates an anagram.
'and' means one lot of letters go next to another.
'cold?' becomes 'c' (eg on taps).
'tango'+'is'='tangois'
'tangois' anagrammed gives 'agnosti'.
'agnosti'+'c'='AGNOSTIC'
(Other definitions for agnostic that I've seen before include "Cost gain for one who holds no spiritual belief" , "One who is undecided" , "Coasting (anag)" , "Don't know" , "I doubt" .)