One succeeding when boxing each amateur (8)
I believe the answer is:
inexpert
'amateur' is the definition.
(I know that amateur can be written as inexpert)
'one succeeding when boxing each' is the wordplay.
'one' becomes 'i' (Roman numeral).
'succeeding' becomes 'next' (next in a sequence).
'when boxing' means one lot of letters goes inside another (inserted letters are boxed in).
'each' becomes 'per' ('per' can mean for each).
'next' going around 'per' is 'nexpert'.
'i'+'nexpert'='INEXPERT'
(Other definitions for inexpert that I've seen before include "Incompetent" , "not clever" , "Bungling" , "not good enough" , "Lacking know-how" .)