Done at once (6)
'done at once' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for immediate that I've seen before include "Prompt,straightaway" , "Prompt, straightaway" , "Involving no delay" , "on the spot" , "Nearest" .)
(Other definitions for prompt that I've seen before include "Cause, bring about" , "Punctual; instigate" , "On the dot" , "Reminder" , "With little or no delay" .)