Covert (6)
'covert' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for secret that I've seen before include "Private, or something private" , "Not generally known" , "Hidden, private" , "Not known by others" , "Private, covert" .)
(Other definitions for ops that I've seen before include "Surgical procedures (abbr)" , "Operations (colloq.)" , "jobs in the theatre" , "facelifts, perhaps" , "medical procedures" .)