Is plum off? I have rash (9)
I believe the answer is:
impulsive
'rash' is the definition.
(rash can mean impulsive or thoughtless)
'is plum off? i have' is the wordplay.
'off?' is an anagram indicator (I've seen 'off' mean this).
'i have' becomes 'I've' (contraction of 'I have').
'is'+'plum'='isplum'
'isplum' anagrammed gives 'impuls'.
'impuls'+'ive'='IMPULSIVE'
(Other definitions for impulsive that I've seen before include "Likely to act without considering the consequences" , "Spontaneous" , "Hot-headed" , "Acting without forethought" , "Done without forethought" .)