Jack Ball at Houston bankers' convenience (4)

I believe the answer is:

john

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'convenience' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for john that I've seen before include "English king in play" , "man" , "Magna Carta king" , "a dear letter" , "Barleycorn, Bull or Doe" .)

I've seen this clue in The Guardian.

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