Spy has no open cases (5)
I believe the answer is:
snoop
'spy' is the definition.
(I know that snoop is a type of spy)
'has no open cases' is the wordplay.
'cases' indicates the answer is hidden within the clue.
'SNOOP' is hidden within 'has no open'.
(Other definitions for snoop that I've seen before include "Stick one's nose in" , "Furtively investigate" , "-- Dogg, US rapper; spy" , "Informally, pry inquisitively" , "Be nosy" .)