Horseman on vacation kept trousers within reach (6)
I believe the answer is:
knight
'horseman' is the definition.
(knight on horseback)
'on vacation kept trousers within reach' is the wordplay.
'on vacation' means to remove the middle letters.
'trousers' means one lot of letters goes inside another (to trouser can mean to steal or take).
'within reach' becomes 'nigh' ('nigh' means near or close by).
'kept' with its middle removed is 'kt'.
'kt' going around 'nigh' is 'KNIGHT'.
(Other definitions for knight that I've seen before include "Mounted warrior of feudal times" , "horse in game!" , "in quest for a mate" , "Chess piece or non-hereditary English peer" , "'Chessman, the horse's head (6)'" .)