A sport (golf) - one amateur's justification (8)
I believe the answer is:
apologia
'justification' is the definition.
(apologia is a kind of justification)
'a sport golf one amateur's' is the wordplay.
'sport' becomes 'polo' (polo is a kind of sport).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'one' becomes 'i' (Roman numeral).
'amateur' becomes 'a'.
'a'+'polo'+'g'+'i'+'a'='APOLOGIA'
(Other definitions for apologia that I've seen before include "Formal defence of one's conduct" , "sort of defence" , "Written defence or vindication" , "Formal defence of one's actions" , "Written defence of one's conduct" .)