Spooky horse seen around hackney carriage? (7)
I believe the answer is:
'spooky' is the definition.
'horse seen around hackney carriage?' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'seen around' means one lot of letters goes inside another.
'hackney carriage?' becomes 'cab' (hackney carriage means a cab or taxi).
'mare' going around 'cab' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Ghastly, gruesome" , "'Grim, repellent (7)'" , "Dark and disturbing" , "Gruesome and weird" , "'Grisly, gruesome (7)'" .)