Duet a star spoilt when soaked (9)
I believe the answer is:
saturated
'soaked' is the definition.
(synonyms)
'duet a star spoilt' is the wordplay.
'spoilt' indicates anagramming the letters.
'duet'+'a'+'star'='duetastar'
'duetastar' with letters rearranged gives 'SATURATED'.
'when' acts as a link.
(Other definitions for saturated that I've seen before include "Drenched" , "dripping" , "Sodden" , "Thoroughly soaked" , "Such fat as may cause disease" .)