Mean boy to become a clergyman (6)
I believe the answer is:
parson
'a clergyman' is the definition.
(parson is a kind of clergyman)
'mean boy' is the wordplay.
'mean' becomes 'par' (I've seen this before).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'par'+'son'='PARSON'
'to become' acts as a link.
(Other definitions for parson that I've seen before include "Anglican clergyman" , "Churchman" , "A rector or vicar" , "Minister of religion" , "service provider" .)