A major, say, providing essential authentication (3,9)
I believe the answer is:
key signature
'a major' is the definition.
(thesaurus)
'providing essential authentication' is the wordplay.
I cannot quite see how this works, but
'essential' could be 'key' (key can mean essential or important) and 'key' is present in the answer.
The remaining letters 'signature' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'say' acts as a link.
Can you help me to learn more?
(Other definitions for key signature that I've seen before include "Musical stave's collection of sharps or flats" , "This fixes pitches" , "Number of sharps or flats on a musical stave" , "What sharps and flats show at start of a music score" , "(In written music) an instruction involving sharps and flats" .)