Linear C surprisingly needing no decryption (2,5)
I believe the answer is:
en clair
'needing no decryption' is the definition.
'linear c surprisingly' is the wordplay.
'surprisingly' indicates an anagram.
'linear'+'c'='linearc'
'linearc' anagrammed gives 'EN CLAIR'.
(Other definitions for en clair that I've seen before include "Not in code or cypher" , "in understandable form" , "Put straightforwardly" , "in ordinary language" , "in plain language" .)