Principal tributary is normal (10)
I believe the answer is:
mainstream
'is normal' is the definition.
(I've seen this before)
'principal tributary' is the wordplay.
'principal' becomes 'main' (both can mean major).
'tributary' becomes 'stream' (I can't explain this - if you can you should believe this answer much more).
'main'+'stream'='MAINSTREAM'
Can you help me to learn more?
(Other definitions for mainstream that I've seen before include "People representing most common ideas" , "Conventional, normal" , "entirely conventional?" , "conventional activities" , "type of jazz" .)