Cloak finally on dark horseman (6)
I believe the answer is:
knight
'horseman' is the definition.
(knight on horseback)
'cloak finally on dark' is the wordplay.
'finally' says to take the final letters.
'on' means one lot of letters go next to another.
'dark' becomes 'night' (it's dark at nighttime).
The last letter of 'cloak' is 'k'.
'k'+'night'='KNIGHT'
(Other definitions for knight that I've seen before include "Chesspiece (in shining armour?)" , "Honour" , "Chevalier" , "Chesspiece - person dubbed" , "soldier" .)