Fish in shallow waters (6)
I believe the answer is:
This could be a double definition.
'fish in shallow' is the first definition.
'shoals' can be an answer for 'shallow' (thesaurus). I am not sure about the 'fish in' bit.
'waters' is the second definition.
(shoal is a kind of water)
Can you help me to learn more?
(Other definitions for shoals that I've seen before include "Masses of fishes swimming together" , "Large groups of fish" .)