Become hot mess following start of summer (7)

I believe the answer is:

swelter

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'become' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for swelter that I've seen before include "Wrestle (anag)" , "Endure oppressive heat" , "Be too hot; Gormenghast chef" , "Sweat and feel faint" , "Feel uncomfortably hot" .)

I've seen this clue in The Telegraph.

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