Minor key adopted by popular worker (6)
I believe the answer is:
infant
'minor' is the definition.
(thesaurus)
'key adopted by popular worker' is the wordplay.
'key' becomes 'f' (I've seen this in other clues**).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'popular' becomes 'in'.
'worker' becomes 'ant' (worker is a type of ant).
'in'+'ant'='inant'
'f' inserted within 'inant' is 'INFANT'.
(Other definitions for infant that I've seen before include "Wee one" , "Rudimentary" , "Babe" , "Toddler" , "Tiny baby" .)