Commanding officer helping guards working in ceremony (10)
I believe the answer is:
coronation
'ceremony' is the definition.
(coronation is a kind of ceremony)
'commanding officer helping guards working' is the wordplay.
'commanding officer' becomes 'co'.
'helping' becomes 'ration' (I've seen this before).
'guards' is an insertion indicator.
'working' becomes 'on'.
'ration' placed around 'on' is 'ronation'.
'co'+'ronation'='CORONATION'
'in' acts as a link.
(Other definitions for coronation that I've seen before include "Royal ceremony of 1953" , "State occasion" , "Ceremony of installing a monarch" , "Enthronement" , "street?" .)