Slow and somewhat sullen, too (5)
I believe the answer is:
lento
'slow' is the definition.
('lento' can be similar in meaning to 'slow')
'somewhat sullen too' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'LENTO' can be found hidden inside 'sullen too'.
'and' acts as a link.
(Other definitions for lento that I've seen before include "Slow passage" , "Musically low speed" , "Slow (music)" , "Slowly (mus.)" , "Scored slowly" .)