17a not a type of fish but another word meaning “to fish” inside (caught) the “banks” of the word TweeD.
18d tricky parsing. Awful poetry is an 8 letter word beginning with the first three letters of your current answer. Replace the latter 4 letters of that 8 letter word with the 3 letter word from the clue itself, and you have a (I’d say more frequently American than British) word for infernal, awful, bad...