Fool mistakenly taints aide (9)
I believe the answer is:
assistant
'aide' is the definition.
(thesaurus)
'fool mistakenly taints' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool).
'mistakenly' is an anagram indicator.
'taints' anagrammed gives 'istant'.
'ass'+'istant'='ASSISTANT'
(Other definitions for assistant that I've seen before include "second" , "Attendant, helper" , "Aide, helper" , "help required?" , "Deputy, subordinate" .)