Still, flat, empty oceans, receding (4,2)
I believe the answer is:
even so
'still' is the definition.
(both can mean nevertheless)
'flat empty oceans receding' is the wordplay.
'flat' becomes 'even' (level).
'empty' says to hollow out the word (remove centre letters).
'receding' says the letters should be written backwards.
'oceans' with its middle removed is 'os'.
'os' back-to-front is 'so'.
'even'+'so'='EVEN SO'
(Other definitions for even so that I've seen before include "Just the same, though that may be the case" , "'Nevertheless (4,2)'" , "anyway" , "yet" , "in any event" .)