Judge with hard heart, one using teeth (7)
I believe the answer is:
arbiter
'judge' is the definition.
(I know that arbiter is a type of judge)
'with hard heart one using teeth' is the wordplay.
'with' means one lot of letters go next to another.
'heart' means to look at the middle letters.
'one using teeth' becomes 'biter' (I've seen this before).
The central letters of 'hard' are 'ar'.
'ar' put next to 'biter' is 'ARBITER'.
(Other definitions for arbiter that I've seen before include "Someone with the power to settle the matter" , "Referee or judge of rarebit" , "Rarebit? (anag)" , "One who settles disputes" , "Judge, final authority" .)