Proper to include son in lineage (7)

I believe the answer is:

descent

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'lineage' is the definition.
(I know that descent is a type of lineage)

'proper to include son in' is the wordplay.
'proper' becomes 'decent' (synonyms).
'to include son' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'decent' placed around 's' is 'DESCENT'.

Can you help me to learn more?

(Other definitions for descent that I've seen before include "Fall" , "Ancestry - downward move" , "A climb down" , "Slope leading from top" , "Downward journey" .)

I've seen this clue in The Times.

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