26 across
Bits
of
grey
matter
in
fish
(10)
Bits of grey matter in fish (10)
hints
0
checks
0
hints
The whole clue is the definition (& lit or double definition).
How the clue works
Here's a nice double definition, no wordplay tricks buried inside. Bits of grey matter gives you the hippocampi, those paired brain structures involved in memory, and separately fish points to hippocampi as the genus name for seahorses. Both readings land on the same word, which is your answer.