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' means one lot of letters goes inside another.
'a minimum of' suggests taking the first letters.
The first 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" .)