My old country's royal event (10)
I believe the answer is:
coronation
'royal event' is the definition.
'my old country's' is the wordplay.
'my' becomes 'cor' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'country' becomes 'nation' ('nation' can be a synonym of 'country').
'cor'+'o'+'nation'='CORONATION'
(Other definitions for coronation that I've seen before include "Crowning" , "Enthronement" , "Royal ceremony of 1953" , "Ceremony of installing a monarch" , "street?" .)