Boy on a register, supporter of the Stuarts (8)
I believe the answer is:
royalist
'supporter of the stuarts' is the definition.
'royalist' can be an answer for 'supporter' (I have seen 'Supporter of monarchy ' mean 'royalist' so perhaps 'supporter' could also mean 'royalist'). I'm not sure about the 'of the stuarts' bit.
'boy on a register' is the wordplay.
'boy on' becomes 'roy' (I can't justify this - if you can you should believe this answer much more).
'register' becomes 'list' (listing is a kind of registering).
'roy'+'a'+'list'='ROYALIST'
Can you help me to learn more?
(Other definitions for royalist that I've seen before include "Solitary kind of person who favours monarchy" , "One pro-monarchy" , "one in civil war" , "Advocate of the principles of monarchy" , "Supporter of monarchy" .)