Joined forces with the boys (5)

I believe the answer is:

sided

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'joined' is the definition.
The answer and definition can be both to do with fundamental existence as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sided that I've seen before include "Showed solidarity (with)" , "Took part with person in a dispute" , "Allied oneself (with one party)" .)

I've seen this clue in the Evening Standard.

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