Not like Iris, dismal when drunk (10)
I believe the answer is:
dissimilar
'not like' is the definition.
('dissimilar' is the opposite to 'like')
'iris dismal when drunk' is the wordplay.
'when drunk' indicates anagramming the letters (I've seen this in other clues).
'iris'+'dismal'='irisdismal'
'irisdismal' with letters rearranged gives 'DISSIMILAR'.
(Other definitions for dissimilar that I've seen before include "Unlike" , "Not like" , "Unalike" .)