Ghost or spy (5)
I believe the answer is:
spook
This clue is a double definition.
'ghost' is the first definition.
(I know that ghost can be written as spook)
'spy' is the second definition.
(spook can informally mean a spy)
(Other definitions for spook that I've seen before include "Startle - spy" , "Enterprise propels him" , "Ghost - US spy" , "Frighten - haunt - spy" , "Ghost, spectre" .)