Crossword Genius

For which one needs to be in fighting form? (5,3)

Ross

I believe the answer is:

class war

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

'for which one needs to be' is the definition.
The definition suggests a singular noun which matches the answer.

'fighting form?' is the wordplay.
I cannot really understand how this works, but
'fighting' could be 'war' and 'war' is located in the answer.
'form?' could be 'class' (synonyms) and 'class' is found within the answer.
No letters remain.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?

(Other definitions for class war that I've seen before include "preached by Lenin?" , "Conflict in society" , "Revolutionary activity" , "Struggle the proletariat is scheduled to win" , "Conflict in Marxist theory" .)

I've seen this clue in The Telegraph, The Irish News and The Guardian.
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