Prevents tooth enamel defects (7)
I believe the answer is:
thwarts
'prevents' is the definition.
(to thwart is to prevent or frustrate)
'tooth enamel defects' is the wordplay.
'enamel' means to remove the middle letters (the outside or \'enamel\' of the word is used).
'defects' becomes 'warts' (wart is a kind of defect).
'tooth' with its centre removed is 'th'.
'th'+'warts'='THWARTS'
(Other definitions for thwarts that I've seen before include "Frustrates" , "Blocks, prevents" .)