Endlessly search for spy (3)
I believe the answer is:
see
'spy' is the definition.
(thesaurus)
'endlessly search' is the wordplay.
'endlessly' means to remove the last letter.
'search' becomes 'seek' ('seek' can be a synonym of 'search').
'seek' with its last letter taken away is 'SEE'.
'for' is the link.
(Other definitions for see that I've seen before include "Look at the bishop's domain" , "Appreciate" , "Jurisdiction of a bishop" , "Rome, notably" , "Realize" .)