Water perhaps over by fish (5)
I believe the answer is:
oxide
'water' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'over by fish' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'by' becomes 'x' (2 by 4 is 2 x 4).
'fish' becomes 'ide' (type of fish).
'o'+'x'+'ide'='OXIDE'
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for oxide that I've seen before include "Compound of oxygen and one other element" , "Rust is iron -" , "Chemical compound" , "Compound of two elements, one with atomic number 8" , "Eg, rust" .)