Ghoulish horse seen around Hackney (7)
I believe the answer is:
macabre
'ghoulish' is the definition.
(thesaurus)
'horse seen around hackney' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'seen around' means one lot of letters goes inside another.
'hackney' becomes 'cab' (I've seen this before).
'mare' going around 'cab' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Gruesome and weird" , "Chilling" , "Gruesome, suggesting death" , "Grotesque" , "gruesome result" .)