Flat seas and oceans primarily will be still (4,2)
I believe the answer is:
even so
'still' is the definition.
(both can mean 'nonetheless')
'flat seas and oceans primarily' is the wordplay.
'flat' becomes 'even' (level).
'seas' becomes 's' (this might be a standard abbreviation of which I'm unaware).
'and' is a charade indicator (letters next to each other).
'primarily' suggests taking the first letters.
The first letter of 'oceans' is 'o'.
'even'+'s'+'o'='EVEN SO'
'will be' is the link.
(Other definitions for even so that I've seen before include "yet" , "however" , "'In spite of that (4,2)'" , "Notwithstanding" , "anyway" .)