Smell or spy (6)
I believe the answer is:
detect
I believe this clue is a double definition.
'smell' is the first definition.
(thesaurus)
'spy' is the second definition.
(I know that detect is a more specific form of spy)
(Other definitions for detect that I've seen before include "Notice, sense" , "Discover, identify" , "Discover the presence of, as Sherlock would" , "Find out like Sherlock" , "Observe, make out" .)