One appearing in knight's gear (3)
I believe the answer is:
kit
'gear' is the definition.
(kit is a kind of gear)
'one appearing in knight's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'appearing in' indicates putting letters inside.
'knight' becomes 'kt' (**).
'i' going into 'kt' is 'KIT'.
(Other definitions for kit that I've seen before include "Colours are visible on such" , "Outfit" , "Set of parts sold to be assembled" , "Football top, shorts, stockings and boots?" , "Young beaver" .)