Out of sorts titaness can be most unpleasant (8)
I believe the answer is:
nastiest
'most unpleasant' is the definition.
('nasty' can be a synonym of 'unpleasant')
'out of sorts titaness' is the wordplay.
'out of sorts' is an anagram indicator.
'titaness' anagrammed gives 'NASTIEST'.
'can be' acts as a link.
(Other definitions for nastiest that I've seen before include "Extremely unpleasant" , "Most revolting" .)