A drudge used to be cagey (7)
I believe the answer is:
guarded
'cagey' is the definition.
(thesaurus)
'a drudge used' is the wordplay.
'used' indicates anagramming the letters.
'a'+'drudge'='adrudge'
'adrudge' anagrammed gives 'GUARDED'.
'to be' acts as a link.
(Other definitions for guarded that I've seen before include "Reticent" , "Cautious, having reservations" , "offered protection" , "Cautious; watched over" , "Very reluctant to give out information" .)