Ruin rolling hill, with decrease in value (8)
I believe the answer is:
downfall
'ruin' is the definition.
(I know that ruin can be written as downfall)
'rolling hill with decrease in value' is the wordplay.
I cannot quite understand how this works, but
'hill' could be 'down' (I've seen this in other clues) and 'down' is found in the answer.
'decrease' could be 'fall' (synonyms) and 'fall' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for downfall that I've seen before include "(Cause of) ruin" , "Loss of power or prosperity, a serious tumble" , "Ruin, humiliation" , "burst of rain" , "state of ruin" .)