Leading priest, one with Joseph's partner (7)

I believe the answer is:

primary

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'leading' is the definition.
(thesaurus)

'priest one with joseph's partner' is the wordplay.
'priest' becomes 'pr'.
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'joseph's partner' becomes 'mary' (I can't justify this - if you can you should believe this answer much more).
'pr'+'i'+'mary'='PRIMARY'

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(Other definitions for primary that I've seen before include "Cardinal" , "Main" , "Principal" , "Paramount" , "Of the first order" .)

I've seen this clue in The Guardian.

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