Black may be so, pocketing a brown (7)
I believe the answer is:
satanic
'black' is the definition.
(thesaurus)
'so pocketing a brown' is the wordplay.
'so' becomes 'sic' ('sic' can be a synonym of 'so').
'pocketing' means one lot of letters goes inside another.
'brown' becomes 'tan' (tan is a kind of brown).
'a'+'tan'='atan'
'sic' enclosing 'atan' is 'SATANIC'.
'may be' acts as a link.
(Other definitions for satanic that I've seen before include "Like hell it is" , "Devil-like" , "wicked" , "like Mephisto" , "Supremely evil" .)