Crossword Genius

Last thing fish will swallow? (4)

Ross

I believe the answer is:

bait

I'm a little stuck... Click here to teach me more about this clue!

'fish will swallow?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'last thing' is the wordplay.
'last' becomes 'ba' (I can't justify this - if you can you should believe this answer much more).
'thing' becomes 'it'.
'ba'+'it'='BAIT'

Can you help me to learn more?

(Other definitions for bait that I've seen before include "Angler's lure for fish" , "Worms, for example" , "Enticement, lure" , "Annoy; enticement" , "N in radio comms" .)

I've seen this clue in The Sun.
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