Mackerel's favourite fish? (5)
I believe the answer is:
sprat
This could be a double definition.
'mackerel's favourite' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'fish?' is the second definition.
(sprat is a kind of fish)
Can you help me to learn more?
(Other definitions for sprat that I've seen before include "Herring's smaller relative" , "Small edible fish" , "Jack --, non-fat eater" , "Small herring-like fish" , "Small sea fish" .)