Offence by Charlie before becoming genuine (7)
I believe the answer is:
sincere
'genuine' is the definition.
('sincere' can be a synonym of 'genuine')
'offence by charlie before' is the wordplay.
'offence' becomes 'sin' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'before' becomes 'ere' (usually literary).
'sin'+'c'+'ere'='SINCERE'
'becoming' acts as a link.
(Other definitions for sincere that I've seen before include "natural?" , "Lacking in deceit" , "like informal correspondents?" , "Open and genuine, not deceitful" , "Genuine; deceit-free" .)