Agents covering ceremony start to realise standards (8)
I believe the answer is:
criteria
'standards' is the definition.
('criterion' can be a synonym of 'standard')
'agents covering ceremony start to realise' is the wordplay.
'agents' becomes 'cia'.
'covering' is an insertion indicator.
'ceremony' becomes 'rite' (rite is a kind of ceremony).
'start to' says to take the initial letters.
The initial letter of 'realise' is 'r'.
'rite'+'r'='riter'
'cia' going around 'riter' is 'CRITERIA'.
(Other definitions for criteria that I've seen before include "Standards for assessment" , "Standards on which to base judgements" , "'Standards, reference points (8)'" , "Standards by which judgement can be made" , "Benchmarks" .)