Warning spirit has been adulterated (7)
I believe the answer is:
banshee
'warning spirit' is the definition.
'banshee' can be an answer for 'spirit' (spirit in Irish mythology). I'm unsure of the 'warning' bit.
'has been adulterated' is the wordplay.
'adulterated' is an anagram indicator (adulterated can mean spoiled).
'has'+'been'='hasbeen'
'hasbeen' with letters rearranged gives 'BANSHEE'.
Can you help me to learn more?
(Other definitions for banshee that I've seen before include "Gaelic spirit" , "'Wailing woman, warns of impending death (7)'" , "Wailing woman of Irish legend" , "Death-portending spirit" , "She predicts death in Ireland" .)