Backward boy, not about to become a chemist (6)
I believe the answer is:
dalton
'a chemist' is the definition.
(Dalton is an example)
'backward boy not about' is the wordplay.
'backward' says the letters should be written backwards.
'boy' becomes 'lad' (informal term for boy).
'about' says the letters should be written in reverse.
'lad' back-to-front is 'dal'.
'not' written backwards gives 'ton'.
'dal'+'ton'='DALTON'
'to become' is the link.
(Other definitions for dalton that I've seen before include "John --, English chemist and physicist" , "English scientist" , "John -, chemist who proposed theory of atoms" , "[LAWMAKER] ?" .)