Commanding officer helping guards working in ceremony (10)
I believe the answer is:
Here is my best explanation:
'ceremony' is the definition.
(coronation is a kind of ceremony)
'commanding officer helping guards working' is the subsidiary indication.
'commanding officer' becomes 'co'.
'helping' becomes 'ration' (I've seen this before**).
'guards' means one lot of letters goes inside another.
'working' becomes 'on'.
'ration' going around 'on' is 'ronation'.
'in' is the link.
(Other definitions for coronation that I've seen before include "Ceremony of installing a monarch", "Crowning" and "Royal ceremony of 1953".)