Small aquarium fish very quietly scoffed by cat (5)
I believe the answer is:
guppy
'small aquarium fish' is the definition.
'guppy' can be an answer for 'fish' (guppy is a kind of fish). I am not sure about the 'small aquarium' bit.
'very quietly scoffed by cat' is the wordplay.
'very quietly' becomes 'pp' (musical abbreviation for pianissimo).
'scoffed by' is an insertion indicator (scoff can mean to eat or consume).
'cat' becomes 'guy' (I can't explain this - if you can you should give a lot more credence to this answer).
'pp' going inside 'guy' is 'GUPPY'.
Can you help me to learn more?
(Other definitions for guppy that I've seen before include "Aquarium fish" , "Tropical fish" , "Freshwater fish" , "Tank occupant" , "Small fish of northern South America and the Caribbean" .)