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

I believe the answer is:

bad dream

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

'tot swallowed by fish' is the wordplay.
'tot' becomes 'add' (synonyms).
'swallowed by' is an insertion indicator.
'fish' becomes 'bream' (type of fish).
'add' inserted inside 'bream' is 'BAD DREAM'.

(Another definition for bad dream that I've seen is " hallucination".)

Want a hint initially instead of a full solution? Install my app