Passable genealogy missing son (6)

I believe the answer is:

decent

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'passable' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for decent that I've seen before include "Nice" , "filling" , "Pretty good - not naked" , "Generous or modest" , "Adequately covered" .)

I've seen this clue in The Guardian.

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