It's a disappointment when dirt won't do so (7)
I believe the answer is:
washout
'so' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it's a disappointment when dirt won't do' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
'do' could be 'ut' (both can mean the first note in a musical scale) and 'ut' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for washout that I've seen before include "Disappointing failure" , "Cancelled event - utter failure" , "match abandoned due to rain?" , "A complete failure - rained off? (4-3)" , "Failure - cancellation" .)