Cover small moor (6)
I believe the answer is:
sheath
'cover' is the definition.
(I know that sheath is a type of cover)
'small moor' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'moor' becomes 'heath' (both can mean wild land).
's'+'heath'='SHEATH'
(Other definitions for sheath that I've seen before include "Scabbard" , "case for weapon" , "Cover for a blade" , "Casing for a sword" , "Protective cover" .)