Tooth's partner (4)
I believe the answer is:
nail
'tooth's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nail that I've seen before include "'For want of a . . . . the shoe was lost'" , "Brad, say" , "Hit this on the head to be dead right" , "A small steel wood pin" , "Metal fastener" .)