Sweet drink's beginning to be commonest contributory factor to rotten teeth (6)
I believe the answer is:
toffee
'sweet' is the definition.
(toffee is a kind of sweet)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for toffee that I've seen before include "Sugar/butter sweet" , "Chewy confection" , "Sticky sweet" , "Chewy stuff" , "Caramel" .)