One thinks it's a bad start when the weather's wet (5)
I believe the answer is:
brain
'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" .)