Referee, endlessly hard, one using teeth (7)
I believe the answer is:
arbiter
'referee' is the definition.
(thesaurus)
'endlessly hard one using teeth' is the wordplay.
'endlessly' says to take the centre (the ends of the words are removed).
'one using teeth' becomes 'biter' (I've seen this before).
The central letters of 'hard' are 'ar'.
'ar'+'biter'='ARBITER'
(Other definitions for arbiter that I've seen before include "One appointed to settle a dispute" , "One judges a disputed issue" , "One settling disputes" , "Referee or judge of rarebit" , "Someone with the power to settle the matter" .)