Dark sort of envelope hosts deliver (9)
I believe the answer is:
saturnine
'dark' is the definition.
(thesaurus)
'sort of envelope hosts deliver' is the wordplay.
'sort of envelope' becomes 'sae' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hosts' means one lot of letters goes inside another.
'deliver' becomes 'turnin' ('turn in' can be a synonym of 'deliver').
'sae' enclosing 'turnin' is 'SATURNINE'.
(Other definitions for saturnine that I've seen before include "dour" , "Gloomy (temperament)" , "A rust nine chnage to be grave and gloomy" , "dull" , "moody" .)