One keeping careful watch in misuse of sugar (5)
I believe the answer is:
argus
'one keeping careful watch' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'misuse of sugar' is the wordplay.
'misuse of' indicates an anagram (I've seen this in other clues).
'sugar' is an anagram of 'ARGUS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for argus that I've seen before include "Hundred-eyed giant in mythology" , "In Greek myth, a many-eyed monster slain by Hermes" , "Giant with one hundred eyes in Greek mythology" , "Monster with many eyes slain by Hermes in Greek myth" , "Pheasant, butterfly, a hundred-eyed monster" .)