Main means of authentication is reduced (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'means of authentication is reduced' is the wordplay.
'means of' becomes 'se' (I am not sure about this - if you are sure you should believe this answer much more).
'is reduced' indicates taking the first letters (the word is reduced to its first letter).
The initial letter of 'authentication' is 'a'.
'se'+'a'='SEA'
Can you help me to learn more?
(Other definitions for sea that I've seen before include "The briny" , "The main" , "Expanse of salt water" , "eg Dead, Red" , "eg Galilee" .)