Area behind a dam (4)
I believe the answer is:
lake
'area behind a dam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lake that I've seen before include "Expanse of water" , "Water in North Italy" , "E.g. Huron" , "Eg, Windermere" , "Mere, tarn" .)