Varied crimes by worker? He must be one (9)
I believe the answer is:
miscreant
'he must be one' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'varied crimes by worker?' is the wordplay.
'varied' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'worker?' becomes 'ant' (worker is a type of ant).
'crimes' is an anagram of 'miscre'.
'miscre'+'ant'='MISCREANT'
(Other definitions for miscreant that I've seen before include "behaving unlawfully" , "criminal" , "culprit" , "Villainous" , "Naughty boy!" .)