Fool ingredient in unwanted company (10)
I believe the answer is:
gooseberry
'unwanted company' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'fool ingredient in' is the wordplay.
'fool' becomes 'goose' (goose is a kind of fool).
'ingredient in' becomes 'berry' (I can't explain this - if you can you should believe this answer much more).
'goose'+'berry'='GOOSEBERRY'
Can you help me to learn more?
(Other definitions for gooseberry that I've seen before include "Fruit - unwanted third person" , "it bears fruit" , "Some intruders play it" , "One unwelcome near passion" , "Third-party" .)