Fighting long duel, Serb discovered key to success (6,4)

I believe the answer is:

golden rule

I'm a little stuck... Click here to teach me more about this clue!

'key to success' is the definition.
I can't judge whether this defines the answer.

'fighting long duel serb discovered' is the wordplay.
'fighting' is an anagram indicator.
'discovered' says to take the centre (dis-covered - outer letters removed).
The centre of 'serb' is 'er'.
'long'+'duel'+'er'='longdueler'
'longdueler' anagrammed gives 'GOLDEN RULE'.

Can you help me to learn more?

(Other definitions for golden rule that I've seen before include "Guiding principle" , "Vital principle" , "Important principle" , "Midas had" , "Basic principle to be followed in all cases" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.