Soft plumage utterly inadequate in heavy rain (8)
I believe the answer is:
downpour
'heavy rain' is the definition.
(I've seen this in another clue)
'soft plumage utterly inadequate' is the wordplay.
'soft plumage' becomes 'down' (I can't explain this - if you can you should give a lot more credence to this answer).
'utterly' indicates a 'sounds like' (homophone) clue (as uttered).
'inadequate' becomes 'poor' (synonyms).
'poor' sounds like 'pour'.
'down'+'pour'='DOWNPOUR'
'in' is the link.
Can you help me to learn more?
(Other definitions for downpour that I've seen before include "Heavy shower" , "Heavy continuous rainfall" , "Deluge" , "Heavy fall of rain" , "falling water" .)