Erratic aunt nicer when drunk (9)
I believe the answer is:
uncertain
'erratic' is the definition.
(thesaurus)
'aunt nicer when drunk' is the wordplay.
'when drunk' indicates an anagram (I've seen this in other clues).
'aunt'+'nicer'='auntnicer'
'auntnicer' with letters rearranged gives 'UNCERTAIN'.
(Other definitions for uncertain that I've seen before include "dubious" , "Unsure" , "Doubtful, indecisive" , "In doubt" , "Not definitely known" .)