Believe it gets rid of flies (7)
I believe the answer is:
swallow
'believe' is the definition.
(swallowing is a kind of believing)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for swallow that I've seen before include "One doesn't make a summer, they say" , "Small migratory bird with forked tail" , "Migrating bird" , "Believe gullibly - bird" , "Fork-tailed bird" .)