Heads of American standards agencies prosecute without delay (4)
I believe the answer is:
asap
'without delay' is the definition.
(as soon as possible)
'heads of american standards agencies prosecute' is the wordplay.
'heads of' suggests taking the first letters.
The first letters of 'american standards agencies prosecute' is 'ASAP'.
(Other definitions for asap that I've seen before include "Common acronym for quick-as-you-can" , "Initials notation meaning with all possible speed" , "Notation meaning with minimum delay" , "Notation-acronym meaning without delay" , "Notation for as quickly as you can" .)