Unwanted third person (10)
I believe the answer is:
gooseberry
'unwanted third person' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gooseberry that I've seen before include "it bears fruit" , "One unwelcome near passion" , "Some intruders play it" , "Third-party" , "Fruit - unwanted third person" .)