Ruling about head of state leaving office (9)
I believe the answer is:
resigning
'leaving office' is the definition.
(resigning is a kind of leaving office)
'ruling about head of' is the wordplay.
'ruling' becomes 'reigning' ('reign' can be a synonym of 'rule').
'about' indicates putting letters inside.
'head of' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'reigning' placed around 's' is 'RESIGNING'.
'state' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for resigning that I've seen before include "Going" , "Giving up one's job" , "throwing up" .)