Terrible rain - friend outside has soak (8)
I believe the answer is:
marinate
'soak' is the definition.
(I know that marinate is a kind of soak)
'terrible rain friend outside' is the wordplay.
'terrible' indicates anagramming the letters.
'friend' becomes 'mate' (mate is a kind of friend).
'outside' means one lot of letters goes inside another (some letters go outside others).
'rain' anagrammed gives 'rina'.
'rina' going into 'mate' is 'MARINATE'.
'has' acts as a link.
(Other definitions for marinate that I've seen before include "Soak in spiced liquid" , "Pickle" , "Soak meat in seasoned liquid before cooking" , "steep in liquor" , "Get soaked" .)