One beset by unusually erratic standards (8)
I believe the answer is:
criteria
'standards' is the definition.
(I know this)
'one beset by unusually erratic' is the wordplay.
'one' becomes 'i' (Roman numeral).
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'unusually' is an anagram indicator.
'erratic' anagrammed gives 'critera'.
'i' going into 'critera' is 'CRITERIA'.
(Other definitions for criteria that I've seen before include "Principles for judging" , "Standards on which to base judgement" , "Standards, yardsticks" , "Tests, yardsticks" , "Standards on which to base judgment" .)