Witch covers knight put to death (4)
I believe the answer is:
hang
'put to death' is the definition.
(I know that hang is a type of put to death)
'witch covers knight' is the wordplay.
'witch' becomes 'hag' (synonyms).
'covers' is an insertion indicator.
'knight' becomes 'N' (chess abbreviation).
'hag' enclosing 'n' is 'HANG'.
(Other definitions for hang that I've seen before include "put up in gallery" , "Suspend (something)" , "Be suspended" , "Kill with a noose" , "Suspend from a height" .)