Tot swallowed by fish — it's a nightmare (3,5)

'it's a nightmare' is the definition.
(I've seen this in another clue)

'tot swallowed by fish aeur' is the wordplay.
I cannot quite understand how this works, but
'tot' could be 'add' (synonyms) and 'add' is found in the answer.
'fish' could be 'bream' (type of fish) and 'bream' is found in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?