Second pound fairy regularly left for this? (5)
I believe the answer is:
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'second pound fairy regularly left' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'pound' becomes 'L' (the pound sign is based on an L).
'regularly left' means one should take alternating letters (letters removed at regular intervals).
The alternating letters of 'fairy' are 'ar'.
'for' is the link.
(Other definitions for molar that I've seen before include "part of the daily grind" , "Chewer and grinder" , "Type of tooth" , "Grinding tooth in the back of a mammal's mouth" , "Moral (anag.)" .)