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