Crowned head is just the thing to bring a page into line (5)
I believe the answer is:
ruler
'crowned head' is the definition.
(I've seen this in another clue)
'just the thing to bring a page into line' is the wordplay.
I cannot really understand how this works, but
'line' could be 'rule' (I've seen this in another clue) and 'rule' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "' - - of the Queen's Navee!'" , "Measuring strip" , "Strip used in drawing straight lines" , "Person commanding" , "measure applied in school?" .)