Ruling knocked up, missing a page (7)
I believe the answer is:
regnant
'ruling' is the definition.
(as in a queen regnant, a queen who rules by birth)
'knocked up missing a page' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for regnant that I've seen before include "Current" , "(Queen) in her own right" , "exercising authority" , "On the throne" , "Not like a consort" .)