There's no catch in it (4,3)
I believe the answer is:
dead sea
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'there's no catch' is the wordplay.
'there's no' becomes 'dead' (I am not sure about this - if you are sure you should believe this answer much more).
'catch' becomes 'sea' (sea fish is a kind of fish).
'dead'+'sea'='DEAD SEA'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for dead sea that I've seen before include "Didn't win; TV series" , "Salt lake fed by the Jordan river" , "Lake of Jordan/Israel" , "River Jordan flows into this" , "Inland salt lake on Israel-Jordan border" .)