Assistant's strange idea on leave (4-2-4)
I believe the answer is:
aide-de-camp
'assistant's' is the definition.
(thesaurus)
'strange idea on leave' is the subsidiary indication.
'strange' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'leave' becomes 'decamp' (decamping is a kind of leaving**).
'idea' with letters rearranged gives 'aide'.
'aide'+'decamp'='aidedecamp'
(Other definitions for aide-de-camp that I've seen before include "Senior officer's assistant" , "Military assistant" , "Military adviser" .)