Bright insects let go some not so bright (9)

I believe the answer is:

fireflies

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'bright insects' is the definition.
'fireflies' can be an answer for 'insects' (firefly is a kind of insect). I am unsure of the 'bright' bit.

'let go some not so bright' is the wordplay.
'let go' becomes 'fire' (both can mean to dismiss from a job).
'some not so bright' becomes 'flies' (I can't explain this - if you can you should believe this answer much more).
'fire'+'flies'='FIREFLIES'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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