Last thing fish will swallow? (4)
I believe the answer is:
bait
'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" .)