One bent on catching a big fish? (3)
I believe the answer is:
rod
'a big fish?' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'one bent on catching' is the wordplay.
'one' becomes 'o' (I've seen this in other clues).
'bent on' becomes 'rd' (I am not sure about this - if you are sure you should believe this answer much more).
'catching' means one lot of letters goes inside another.
'o' put within 'rd' is 'ROD'.
Can you help me to learn more?
(Other definitions for rod that I've seen before include "Fishing implement" , "Spare it and spoil the child, they used to say" , "Flexible stick for 11 Down" , "Little boy" , "Staff" .)