A cola is unfortunately not a mixer (7)
I believe the answer is:
asocial
'not a mixer' is the definition.
I can't tell whether this definition defines the answer.
'a cola is unfortunately' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'cola'+'is'='colais'
'colais' is an anagram of 'social'.
'a'+'social'='ASOCIAL'
Can you help me to learn more?
(Other definitions for asocial that I've seen before include "Solitary" , "lacking friendly instincts" , "not connected with community" , "Not gregarious" , "Not fond of company, perhaps inconsiderate" .)