Fly, perhaps, one swallowed by nocturnal insectivore (4)
I believe the answer is:
bait
'fly perhaps' is the definition.
(fly is a kind of bait)
'one swallowed by nocturnal insectivore' is the wordplay.
'one' becomes 'i' (Roman numeral).
'swallowed by' indicates putting letters inside.
'nocturnal insectivore' becomes 'bat' (I can't explain this - if you can you should believe this answer much more).
'i' put into 'bat' is 'BAIT'.
Can you help me to learn more?
(Other definitions for bait that I've seen before include "A lure to catch fish" , "Enticement for fish" , "Boilie, for example" , "What lures" , "Food to lure fish" .)