There's always one on the other side (5)

I believe the answer is:

enemy

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'one on the other side' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for enemy that I've seen before include "one in opposition" , "is against us" , "Hostile person, foe" , "Hostile opponent" , "Antagonist" .)

I've seen this clue in the Evening Standard.

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