Weaken report of reservoir depletion (5,4)

I believe the answer is:

water down

I'm a little stuck... Click here to teach me more about this clue!

'weaken' is the definition.
(I know that water down is a more specific form of the action cut)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for water down that I've seen before include "Lessen impact" , "Dilute - make less controversial" , "Dilute by adding H20" , "Dilute, make less forceful or vivid" .)

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app