Petulant doctor unknown to drink spirit (6)
I believe the answer is:
grumpy
'petulant' is the definition.
(thesaurus)
'doctor unknown to drink spirit' is the wordplay.
'doctor' becomes 'gp' (general practitioner).
'unknown' becomes 'y' (algebra).
'to drink' means one lot of letters goes inside another.
'spirit' becomes 'rum' (alcoholic spirit).
'gp'+'y'='gpy'
'gpy' enclosing 'rum' is 'GRUMPY'.
(Other definitions for grumpy that I've seen before include "likely to snap?" , "Gloomy and irritable like sour old man" , "The cantankerous Dwarf" , "Bad-tempered and out of sorts, like one of the Seven Dwarfs" , "Churlish" .)