Special gin in cocktail? This might be a plus (4)
I believe the answer is:
sign
'this might be a plus' is the definition.
(plus sign is a type of sign)
'special gin in cocktail?' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'in cocktail?' indicates anagramming the letters (letters shaken up like a cocktail).
'gin' is an anagram of 'ign'.
's'+'ign'='SIGN'
(Other definitions for sign that I've seen before include "Communicate to the deaf" , "Public display of a message" , "Indicator" , "Public notice; portent" , "Notice; omen" .)