Second in respectable lineage (7)
I believe the answer is:
descent
'lineage' is the definition.
(I know that lineage can be written as descent)
'second in respectable' is the wordplay.
'second' becomes 's' (common abbreviation).
'in' is an insertion indicator.
'respectable' becomes 'decent' (similar in meaning).
's' put into 'decent' is 'DESCENT'.
(Other definitions for descent that I've seen before include "Downward slope" , "Lineage" , "Drop" , "Coming down" , "Slope leading from top" .)