Stuck-up favourite son, more remote, losing right to be family member (10)
I believe the answer is:
stepmother
'be family member' is the definition.
(I've seen this in another clue)
'stuck-up favourite son more remote losing right' is the wordplay.
'stuck up' shows that the letters should be reversed in order.
'favourite' becomes 'pet' (synonyms).
'son' becomes 's' (genealogical abbreviation for son).
'more remote losing right' becomes 'mother' (I am not sure about this - if you are sure you should believe this answer much more).
'pet'+'s'='pets'
'pets' written backwards gives 'step'.
'step'+'mother'='STEPMOTHER'
'to' is the link.
Can you help me to learn more?
(Other definitions for stepmother that I've seen before include "A relation by marriage" , "One proverbially cruel" , "Relative by a further marriage of one's father" , "Wicked character of fairy stories" , "Unnatural parent" .)