Popular aquarium fish very quietly swallowed by a cat (5)
I believe the answer is:
guppy
'popular aquarium fish' is the definition.
(I've seen this in another clue)
'very quietly swallowed by a cat' is the wordplay.
'very quietly' becomes 'pp' (musical abbreviation for pianissimo).
'swallowed by' indicates putting letters inside.
'a cat' becomes 'guy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pp' put within 'guy' is 'GUPPY'.
Can you help me to learn more?
(Other definitions for guppy that I've seen before include "Small bright WI fish" , "Freshwater fish" , "Aquarium fish" , "Tropical fish" , "Small fish of northern South America and the Caribbean" .)