Rain mixed into manufactured sauce (8)
I believe the answer is:
marinade
'sauce' is the definition.
(I know that marinade is a type of sauce)
'rain mixed into manufactured' is the wordplay.
'mixed' indicates anagramming the letters.
'into' means one lot of letters goes inside another.
'manufactured' becomes 'made' (I've seen this in another clue).
'rain' anagrammed gives 'rina'.
'rina' going into 'made' is 'MARINADE'.
(Other definitions for marinade that I've seen before include "Mixture for soaking food before cooking" , "get saturated" , "pickle" , "Food is steeped in this to improve flavour" , "Steep in seasoning" .)