Fish in a London well! (4)
I believe the answer is:
shad
'fish' is the definition.
(shad is a kind of fish)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for shad that I've seen before include "fish that's been caught" , "Fish; dash (anag.)" , "Herring-like fish" , "Fish related to the herring" .)