Single person who may be duped immediately (4)
I believe the answer is:
asap
'immediately' is the definition.
(I've seen this in another clue)
'single person who may be duped' is the wordplay.
'single' becomes 'a' ().
'person who may be duped' becomes 'sap' (sap can mean an easily duped person).
'a'+'sap'='ASAP'
(Other definitions for asap that I've seen before include "As soon as you can, initially" , "Notation for urgent, quick as you can (4)" , "As soon as possible (abbrev.)" , "Notation-acronym meaning without delay" , "Notation for as quickly as you can" .)