If the order is not the usual he'll help himself (5)
I believe the answer is:
thief
'not the usual he'll help himself' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'if the order' is the wordplay.
'order' indicates an anagram.
'if'+'the'='ifthe'
'ifthe' is an anagram of 'THIEF'.
'is' is the link.
Can you help me to learn more?
(Other definitions for thief that I've seen before include "nicker?" , "Crook" , "Aus. or Scots city" , "He takes" , "See 3" .)