Something fishy that won't hold water (3)
I believe the answer is:
net
'something fishy' is the definition.
(netting fish is a kind of fishing)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for net that I've seen before include "difficulty" , "Mesh for the bottom line?" , "Cricket practice area" , "Make" , "After expenses" .)