Covering on tip of the finger (4)
I believe the answer is:
nail
'covering on tip of the finger' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nail that I've seen before include "Hit this on the head to be bang-on" , "Horny covering of the digits" , "Perfectly execute" , "Apprehend" , "Hard plate at end of digit" .)