Nothing covering a sharp object (4)
I believe the answer is:
nail
'sharp object' is the definition.
(a nail is a sharp, pointed object)
'nothing covering a' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'covering' is an insertion indicator.
'nil' going around 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Carpentry fastener" , "Hit this on the head to be bang on" , "for the carpenter?" , "Fastening device" , "Thin pointed fastener" .)