Force fighting rebel group (7)

I believe the answer is:

faction

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'rebel group' is the definition.
'faction' can be an answer for 'group'. I'm not certain of the 'rebel' bit.

'force fighting' is the wordplay.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'fighting' becomes 'action' (I've seen this before).
'f'+'action'='FACTION'

Can you help me to learn more?

(Other definitions for faction that I've seen before include "group of rebels" , "camp" , "Small trouble-making group within a group" , "Rebellious group" , "Act on if for a small group within a larger one" .)

I've seen this clue in The Sun.

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