Start aiming straight at prey, without delay! (4)
I believe the answer is:
asap
'without delay' is the definition.
(as soon as possible)
'start aiming straight at prey' is the wordplay.
'start' says to take the initial letters.
The first letters of 'aiming straight at prey' is 'ASAP'.
(Other definitions for asap that I've seen before include "Notation for urgent, quick as you can (4)" , "At one's earliest convenience" , "As soon as you can, initially" , "Notation or acronym meaning without avoidable delay" , "'At the earliest moment, initially (4)'" .)