Crossword Genius

One thinks it's a bad start when the weather's wet (5)

Ross

I believe the answer is:

brain

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

'one thinks it's a bad start when' is the definition.
I can't judge whether this definition defines the answer.

'weather's wet' is the wordplay.
'weather's' becomes 'b' (I can't justify this - if you can you should give a lot more credence to this answer).
'wet' becomes 'rain' (wet weather).
'b'+'rain'='BRAIN'

'the' acts as a link.

Can you help me to learn more?

(Other definitions for brain that I've seen before include "to get hit over the head" , "It's in the skull" , "Whack on the head" , "Thinking thing" , "Encephalon" .)

I've seen this clue in the Evening Standard.
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