Younger family-member treads upon it (7)
I believe the answer is:
stepson
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'younger family-member treads upon' is the wordplay.
'younger family member' becomes 'on' (I can't justify this - if you can you should believe this answer much more).
'treads' becomes 'steps' ('step' can be a synonym of 'tread').
'upon' says to put letters next to each other.
'on' put after 'steps' is 'STEPSON'.
Can you help me to learn more?
(Other definitions for stepson that I've seen before include "No pests about this relation by marriage" , "Spouse's child from previous marriage" , "Tramples (5,2) -- relative" , "No pests with relative by marriage" , "Child from a spouse's former marriage" .)