Damage in the back part of tooth (7)
I believe the answer is:
dentine
'part of tooth' is the definition.
('dentine' is part of 'tooth')
'damage in the back' is the wordplay.
'damage' becomes 'dent' (dent is a kind of damage).
'back' says to take the final letters (the letter at the back).
The final letter of 'the' is 'e'.
'dent'+'in'+'e'='DENTINE'
(Other definitions for dentine that I've seen before include "Tooth material" , "Tin need (anag.)" , "bony material" , "Material in teeth" , "which forms tooth" .)