Flying back on time in summer, dropping from the sky? (5)

I believe the answer is:

guano

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'flying back on time in' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for guano that I've seen before include "Touching month turning over" , "MP's first address to House" , "business to aid growth" , "Bird droppings used as fertiliser" , "Sea bird droppings" .)

I've seen this clue in The Times.

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