Kind of tart whose company’s not wanted (10)
I believe the answer is:
gooseberry
This clue is a double definition.
'kind of tart' is the first definition.
(as in a gooseberry tart)
'whose company's not wanted' is the second definition.
(term for a person getting in the way of a couple)
(Other definitions for gooseberry that I've seen before include "Third-party" , "One unwelcome near passion" , "Fruit - unwanted third person" , "Some intruders play it" , "it bears fruit" .)