Wine from flooded valley comprising one jeroboam originally (5)
I believe the answer is:
rioja
'wine' is the definition.
(Rioja is an example)
'flooded valley comprising one jeroboam originally' is the wordplay.
'flooded valley' becomes 'ria' (I've seen this before).
'comprising' indicates putting letters inside.
'originally' indicates taking the first letters.
The first letters of 'one jeroboam' is 'oj'.
'ria' placed around 'oj' is 'RIOJA'.
'from' acts as a link.
(Other definitions for rioja that I've seen before include "it comes from grapevine" , "kind of red" , "Spanish table wine" , "North Spanish wine" , "Dry red wine from Spain" .)