Old enough to finally smoke drugs (2,3)
I believe the answer is:
of age
'old enough' is the definition.
(having come of age)
'to finally smoke drugs' is the wordplay.
'finally' says to take the final letters.
'smoke' becomes 'fag' (both can mean a cigarette).
'drugs' becomes 'E' (somebody 'on E' could also be 'on drugs').
The final letter of 'to' is 'o'.
'o'+'fag'+'e'='OF AGE'
(Other definitions for of age that I've seen before include "Having adult legal rights" , "Considered adult, able to vote etc" , "In maturity" , "Legally adult, able to vote" , "Principally, old enough" .)