Swimmers that hiss as they listen in? (6)
I believe the answer is:
sharks
'swimmers that hiss' is the definition.
'sharks' can be an answer for 'swimmers' (shark is a kind of aquatic vertebrate). I am unsure of the remainder of the definition.
'they listen in?' is the wordplay.
'they' becomes 'ss' (I can't justify this - if you can you should believe this answer much more).
'listen' becomes 'hark' (harking is a kind of listening).
'in?' means one lot of letters goes inside another.
'ss' going around 'hark' is 'SHARKS'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for sharks that I've seen before include "Large carnivorous fish" , "Voracious sea fish" , "Feared fishes, may be basking or hammerhead" .)