A mug served up cold (5)
I believe the answer is:
aloof
'cold' is the definition.
(synonyms)
'a mug served up' is the wordplay.
'mug' becomes 'fool' (synonyms).
'served up' says the letters should be written backwards.
'fool' reversed gives 'loof'.
'a'+'loof'='ALOOF'
(Other definitions for aloof that I've seen before include "Socially detached" , "Cool, distant" , "Snooty" , "Disinterested" , "Standing apart" .)