Big boxer stumped believer in Murphy's Law? (8)
I believe the answer is:
fatalist
'believer in murphy's law?' is the definition.
(Murphy's Law is a fatalist approach to life)
'big boxer stumped' is the wordplay.
'big' becomes 'fat'.
'boxer' becomes 'ali' (Muhammad Ali).
'stumped' becomes 'st' (cricket abbreviation).
'fat'+'ali'+'st'='FATALIST'
(Other definitions for fatalist that I've seen before include "One with a submissive attitude to events" , "Predestinarian" , "One who accepts whatever happens as inevitable" , "philosophical type" , "One submitting to destiny" .)