Although friendly, a chap fibbed (6)
I believe the answer is:
allied
'although friendly a' is the definition.
I can't tell whether this defines the answer.
'chap fibbed' is the wordplay.
'chap' becomes 'Al' (man's name).
'fibbed' becomes 'lied' (fib can mean to tell a lie).
'al'+'lied'='ALLIED'
Can you help me to learn more?
(Other definitions for allied that I've seen before include "Joined together for a purpose" , "Joined for mutual benefit" , "United through treaty" , "Joined by, eg, treaty" , "in concert" .)