Singer starts to truly hate prog band (6)
I believe the answer is:
thrush
'singer' is the definition.
(type of songbird)
'starts to truly hate prog band' is the wordplay.
'starts to' suggests taking the first letters.
'prog band' becomes 'rush' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letters of 'truly hate' is 'th'.
'th'+'rush'='THRUSH'
Can you help me to learn more?
(Other definitions for thrush that I've seen before include "Common British songbird" , "Repetitive singer" , "Speckled songbird or throat infection" , "sprue" , "Mouth fungus or songbird" .)