One thinking to start back when the weather's wet? (5)
I believe the answer is:
brain
'one thinking to start back when the weather's wet?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one thinking to start back' is the wordplay.
'one thinking' becomes 'rain' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'start' indicates taking the first letters.
The first letter of 'back' is 'b'.
'rain' put after 'b' is 'BRAIN'.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Organ of the intellect" , "Thinking thing" , "Hit hard; intellect" , "Encephalon" , "The grey matter" .)