Fighter backing king of Britain at outset (5)
I believe the answer is:
boxer
'fighter' is the definition.
(I know that boxer is a type of fighter)
'backing king of britain at outset' is the wordplay.
'backing' says the letters should be written in reverse.
'king' becomes 'rex' ('king' in Latin).
'at outset' indicates taking the first letters.
The initial letters of 'of britain' is 'ob'.
'rex'+'ob'='rexob'
'rexob' backwards is 'BOXER'.
(Other definitions for boxer that I've seen before include "canine" , "one that's accustomed to fighting" , "Breed of stocky short-haired dog" , "Tyson, perhaps - type of dog" , "Dog - fighter" .)