Old royal family writer move very slowly? (9)
I believe the answer is:
pendragon
'old royal family' is the definition.
The definition suggests a singular noun which matches the answer.
'writer move very slowly?' is the wordplay.
'writer' becomes 'pen' (something which writes).
'move very slowly?' becomes 'dragon' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pen'+'dragon'='PENDRAGON'
Can you help me to learn more?
(Other definitions for pendragon that I've seen before include "Ruler" , "Uther --" , "Tribal chief," , "old British chief" , "King Arthur, for example" .)