Dry, only letting in a minimum of rain (4)
I believe the answer is:
brut
'dry' is the definition.
(type of wine)
'only letting in a minimum of rain' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'letting in' indicates putting letters inside.
'a minimum of' says to take the initial letters.
The initial letter of 'rain' is 'r'.
'but' placed around 'r' is 'BRUT'.
(Other definitions for brut that I've seen before include "(Of sparkling wines) very dry" , "(Of sparkling wine) extremely dry" , "Dry (of champagne)" , "Unsweetened; very dry" , "one type of champagne" .)