It’s a disappointment when the colours aren’t fast (7)
I believe the answer is:
washout
'fast' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for washout that I've seen before include "Cancelled test, for example" , "A complete failure - too much rain" , "Cancelled event - utter failure" , "A complete failure - rained off? (4-3)" , "match abandoned due to rain?" .)