Coin is breaking ringleader's tooth (7)
I believe the answer is:
incisor
'tooth' is the definition.
(I know that incisor is a type of tooth)
'coin is breaking ringleader's' is the wordplay.
'breaking' is an anagram indicator.
'ringleader' becomes 'r' (first letter of ring).
'coin'+'is'='coinis'
'coinis' anagrammed gives 'inciso'.
'inciso'+'r'='INCISOR'
(Other definitions for incisor that I've seen before include "One takes a bite" , "Cutting tooth" , "oral equipment" , "companion could be Goofy" , "Chisel-edged tooth" .)