Constant smoker somewhat sweet-natured (4)
I believe the answer is:
etna
'constant smoker' is the definition.
(I've seen this before)
'somewhat sweet-natured' is the wordplay.
'somewhat' indicates a hidden word.
'ETNA' is hidden in the letters of 'sweet natured'.
(Other definitions for etna that I've seen before include "From a dangerous height" , "Volcano; reverse of stake" , "one blowing top?" , "Sicilian volcano" , "feature spewing lava" .)