Average boy becomes minister (6)
I believe the answer is:
parson
'minister' is the definition.
(church minister)
'average boy' is the wordplay.
'average' becomes 'par' (average score in golf).
'boy' becomes 'son' (male child).
'par'+'son'='PARSON'
'becomes' acts as a link.
(Other definitions for parson that I've seen before include "A rector or vicar" , "Aprons (anag.)" , "Rector; clergyman" , "Churchman" , "man of the church" .)