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 understand 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 present in the answer.
'do' could be 'ut' (both can mean the first note in a musical scale) and 'ut' is found in the answer.
This explanation may well be incorrect...
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" .)