Rider lives in High Street as part of extended family (10)
I believe the answer is:
stepsister
'part of extended family' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'rider lives in high street' is the wordplay.
'rider' becomes 'PS' (a rider is a clause added to a contract, so a P.S.).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'in' means one lot of letters goes inside another.
'high' indicates anagramming the letters (like drunk).
'ps'+'is'='psis'
'street' is an anagram of 'steter'.
'psis' put into 'steter' is 'STEPSISTER'.
'as' is the link.
Can you help me to learn more?
(Other definitions for stepsister that I've seen before include "member of the family" , "Relation gained through a second marriage" , "relative in law" , "one who hated Cinderella" .)