Come by secret code protecting procedure (3,2)
I believe the answer is:
pop in
'come by' is the definition.
(thesaurus)
'secret code protecting procedure' is the wordplay.
'secret code' becomes 'pin' (personal identification number).
'protecting' is an insertion indicator (inserted letters are held or protected).
'procedure' becomes 'op' (short for surgical operation).
'pin' enclosing 'op' is 'POP IN'.
(Other definitions for pop in that I've seen before include "Call" , "Pay a quick informal visit" , "Visit briefly" .)