I fish, letting nothing escape, in area with a coast (4)
I believe the answer is:
isle
'letting nothing escape in area with a coast' is the definition.
I can't tell whether this definition defines the answer.
'i fish' is the wordplay.
'fish' becomes 'sle' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'sle'='ISLE'
Can you help me to learn more?
(Other definitions for isle that I've seen before include "Man, maybe" , "Man, Wight, etc" , "Land encircled by water" , "Small region of land" , "Eg, eyot or inch" .)