Knack is a secret (5)
I believe the answer is:
trick
This clue is a double definition.
'knack' is the first definition.
(thesaurus)
'a secret' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for trick that I've seen before include "Dupe; ruse" , "Deceive, perhaps with cards" , "Deceitful device" , "Deception or illusion" , "Point gained in bridge - a deception?" .)