Net used in some shark-infested waters (4)
I believe the answer is:
mesh
'net' is the definition.
(I know that net can be written as mesh)
'some shark-infested waters' is the wordplay.
'some' says to take the initial letters.
'infested' indicates putting letters inside (some letters are infested with others).
'waters' becomes 'meh' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'shark' is 's'.
's' put inside 'meh' is 'MESH'.
'used in' acts as a link.
Can you help me to learn more?
(Other definitions for mesh that I've seen before include "fit together" , "Net, snare" , "Net fabric" , "Trap" , "Crossed wires" .)