Found inside cake, note needing no decryption (2,5)
I believe the answer is:
en clair
'needing no decryption' is the definition.
'found inside cake note' is the wordplay.
'found inside' is an insertion indicator.
'cake' becomes 'eclair' (eclair is a type of cake).
'note' becomes 'n' (abbreviation).
'eclair' going around 'n' is 'EN CLAIR'.
(Other definitions for en clair that I've seen before include "Put straightforwardly" , "in understandable form" , "in ordinary language" , "in common parlance" , "in plain language" .)