Son will be taken in by respectable lineage (7)
I believe the answer is:
descent
'lineage' is the definition.
(descent is a kind of lineage)
'son will be taken in by respectable' is the wordplay.
'son' becomes 's'.
'will be taken in by' means one lot of letters goes inside another.
'respectable' becomes 'decent' (similar in meaning).
's' placed within 'decent' is 'DESCENT'.
(Other definitions for descent that I've seen before include "A climb down" , "Bloodline" , "Downward path" , "Pedigree" , "Ancestry - downward move" .)