The rain will spoil a flimsy top cover (7)

I believe the answer is:

hairnet

I'm a little stuck... Click here to teach me more about this clue!

'top cover' is the definition.

'the rain will spoil a flimsy' is the wordplay.
'the rain' can be anagrammed to 'HAIRNET'.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for hairnet that I've seen before include "In heart (anag)" , "Tonsorial restraint" , "Lock-keeper" , "cause of static shock?" , "keeper of locks" .)

I've seen this clue in The Irish News.

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