One reduces friction in party with relation (4,7)

I believe the answer is:

ball bearing

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'one reduces friction' is the definition.
I can't judge whether this defines the answer.

'party with relation' is the wordplay.
'party' becomes 'ball' (ball is a kind of party).
'with' is a charade indicator (letters next to each other).
'relation' becomes 'bearing' (bearing is a kind of relation).
'ball'+'bearing'='BALL-BEARING'

'in' is the link.

Can you help me to learn more?

(Another definition for ball bearing that I've seen is " one in a race".)

I've seen this clue in The Guardian.

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