Sons featuring in proper family tree (7)
I believe the answer is:
descent
'family tree' is the definition.
(I know that descent is a type of family tree)
'sons featuring in proper' is the wordplay.
'sons' becomes 's'.
'featuring in' means one lot of letters goes inside another.
'proper' becomes 'decent' (associated in meaning).
's' placed inside 'decent' is 'DESCENT'.
(Other definitions for descent that I've seen before include "A climb down" , "Pedigree" , "Drop" , "Decline" , "Downward journey" .)