Occasionally I seem most unpleasant (9)
I believe the answer is:
sometimes
'occasionally' is the definition.
(I've seen this before)
'i seem most unpleasant' is the wordplay.
'unpleasant' indicates anagramming the letters.
'i'+'seem'+'most'='iseemmost'
'iseemmost' with letters rearranged gives 'SOMETIMES'.
(Other definitions for sometimes that I've seen before include "Seem moist (anag.)" , "Occasionally available" , "On occasion" , "Not always" .)