Erratic individual mixed up the standards by which it should be decided (8)
I believe the answer is:
criteria
'standards by which it should be decided' is the definition.
'criteria' can be an answer for 'standards' ('criterion' can be a synonym of 'standard'). I am not sure about the remainder of the definition.
'erratic individual mixed up' is the wordplay.
'individual' becomes 'i' (Roman numeral for one).
'mixed up' is an anagram indicator.
'erratic'+'i'='erratici'
'erratici' with letters rearranged gives 'CRITERIA'.
'the' is the link.
Can you help me to learn more?
(Other definitions for criteria that I've seen before include "Standards on which to base judgments" , "Standards for assessment" , "Standards by which things measured" , "Standards on which to base judgment" , "Standards on which to base judgements" .)