Sea essentially still and flat (11)
I believe the answer is:
emotionless
'flat' is the definition.
(synonyms)
'sea essentially still' is the wordplay.
'essentially' indicates the central letters.
'still' becomes 'motionless' (synonyms).
The middle of 'sea' is 'e'.
'e'+'motionless'='EMOTIONLESS'
'and' is the link.
(Other definitions for emotionless that I've seen before include "devoid of passion" , "lacking passion" , "Unmoved by feelings" , "cold and hard?" , "Unfeeling" .)