Military assistant's funny idea -- leg it! (4-2-4)
I believe the answer is:
aide-de-camp
'military assistant's' is the definition.
(I've seen this in another clue)
'funny idea leg it' is the wordplay.
'funny' indicates anagramming the letters.
'leg it' becomes 'decamp' (to decamp is to leave or depart).
'idea' with letters rearranged gives 'aide'.
'aide'+'decamp'='AIDE-DE-CAMP'
(Other definitions for aide-de-camp that I've seen before include "Military assistant" , "Military adviser" , "Senior officer's assistant" .)