Something disappointing — relegation? That has sour taste? (5,4)
I believe the answer is:
lemon drop
'sour taste?' is the definition.
The definition suggests a singular noun which matches the answer.
'something disappointing relegation? that has' is the wordplay.
'something disappointing' becomes 'lemon' (lemon can mean a faulty or unsatisfactory thing).
'relegation?' becomes 'drop' (relegation can mean dropping down).
'that has' means one lot of letters go next to another.
'lemon'+'drop'='LEMON-DROP'
Can you help me to learn more?
(Another definition for lemon drop that I've seen is " Fruit sweet".)