Black chest in study (6)
I believe the answer is:
darken
'black' is the definition.
(thesaurus)
'chest in study' is the wordplay.
'chest' becomes 'ark' (historical term for a chest).
'in' means one lot of letters goes inside another.
'study' becomes 'den' (I've seen this before).
'ark' placed into 'den' is 'DARKEN'.
(Other definitions for darken that I've seen before include "Cloud over" , "shade" , "Dim" , "Cast a shadow over" , "Grow less light" .)