Couple almost knocked over relative in troublesome spot (9)
I believe the answer is:
'troublesome spot' is the definition.
(spot or sore on the body)
'couple almost knocked over relative' is the wordplay.
I cannot really see how this works, but
'relative' could be 'uncle' (uncle is a kind of relative) and 'uncle' is found within the answer.
The remaining letters 'carb' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
(Other definitions for carbuncle that I've seen before include "Architectural eyesore" , "Red gemstone; sort of boil" , "National Gallery's Sainsbury Wing?" , "Sort of boil; a gem" , "Skin eruption - gemstone" .)