Ruling about head of state leaving office (9)
I believe the answer is:
Here is my best explanation:
'leaving office' is the definition.
(resigning is a kind of leaving office)
'ruling about head of state' is the subsidiary indication.
'ruling' becomes 'reigning' (reigning is a kind of ruling).
'about' is an insertion indicator.
'head of' indicates taking the first letters.
The initial letter of 'state' is 's'.
'reigning' going around 's' is 'resigning'.
(Other definitions for resigning that I've seen before include "Going", "Giving up one's job" and "throwing up".)