Sea area with fish, not without duck (6)
I believe the answer is:
solent
'sea area' is the definition.
The definition suggests a singular noun which matches the answer.
'fish not without duck' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish).
'without' is a deletion indicator.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'not' with 'o' taken out is 'nt'.
'sole'+'nt'='SOLENT'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for solent that I've seen before include "Channel N. of IOW" , "Channel separating Hampshire from the Isle of Wight" , "stretch of water" , "sound" , "South coast channel" .)