Fighter beginning to back-pedal, one in defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'fighter beginning to back-pedal one' is the wordplay.
'fighter' becomes 'ali' (I have seen 'Old fighter' mean 'ali' so perhaps 'fighter' could also mean 'ali').
'beginning to' says to take the initial letters.
'pedal one' becomes 'i' (Roman numeral. I am not sure about the 'pedal' bit.**).
The initial letter of 'back' is 'b'.
'ali'+'b'+'i'='ALIBI'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Accused person's defence" , "Defence against accusation - was somewhere else" , "Evidence one was not at crime scene" , "Defence in court" , "Legal term from the Latin 'elsewhere'" .)