Assistant’s terrible idea to do a runner (4-2-4)
I believe the answer is:
aide-de-camp
'assistant's' is the definition.
(I've seen this before)
'terrible idea to do a runner' is the wordplay.
'terrible' is an anagram indicator.
'to do a runner' becomes 'decamp' (I've seen this before).
'idea' is an anagram of 'aide'.
'aide'+'decamp'='AIDE-DE-CAMP'
(Other definitions for aide-de-camp that I've seen before include "Military assistant" , "Senior officer's assistant" , "Military adviser" .)