Group of revolutionary old soldiers (3,8)

I believe the answer is:

foo fighters

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

'revolutionary old soldiers' is the definition.
I know nothing about this answer so I can't judge whether this works.

'group of' is the wordplay.
I cannot quite understand how this works, but
'group' could be 'o' (O is an example) and 'o' is present in the answer.
'of' could be 'o' and 'o' is found within the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.

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