Sound of ignominious defeat, unholy alliance originally having done U-turn (5)
I believe the answer is:
audio
'sound' is the definition.
(audio means sound)
'of ignominious defeat unholy alliance originally having done u-turn' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'originally' says to take the initial letters.
'having' means one lot of letters go next to another.
'done u-turn' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'ignominious defeat unholy alliance' is 'idua'.
'idua' anagrammed gives 'audi'.
'audi'+'o'='AUDIO'
Can you help me to learn more?
(Other definitions for audio that I've seen before include "Prefix signifying sound or hearing" , "Television sound" , "Denoting sound" , "Electronic sound reproduction" , "Prefix indicating hearing or sound" .)