Before you do anything else (4)
I believe the answer is:
asap
'before you do anything else' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for asap that I've seen before include "Notation or acronym meaning without avoidable delay" , "Common acronym for quick-as-you-can" , "Notation for urgent, quick as you can (4)" , "Notation meaning without delay, fast as you can" , "As soon as you can, initially" .)