Bridge partner, the so-called "fourth" (5)

I believe the answer is:

forth

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'bridge partner the' is the definition.
I can't judge whether this defines the answer.

'so-called fourth' is the wordplay.
'so-called' shows a homophone (sound like).
'fourth' sounds like 'FORTH'.

Can you help me to learn more?

(Other definitions for forth that I've seen before include "(Go) away" , "River and estuary in eastern Scotland" , "Scottish river and firth" , "Onward direction" , "Water" .)

I've seen this clue in the Evening Standard.

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