A spot of wet weather (8)
I believe the answer is:
raindrop
'a spot' is the definition.
(I have seen 'Spot of precipitation' mean 'raindrop' so perhaps 'spot' could also mean 'raindrop')
'wet weather' is the wordplay.
'wet' becomes 'rain' (wet weather).
'weather' becomes 'drop' (I can't explain this - if you can you should give a lot more credence to this answer).
'rain'+'drop'='RAINDROP'
'of' is the link.
Can you help me to learn more?
(Other definitions for raindrop that I've seen before include "Little evidence of wet weather" , "tiny amount of water" , "Bit of precipitation" , "Precipitated globule - keeps falling on my head in song" , "bit of the wet stuff" .)