Solitary in tree? Rubbish! (7)
I believe the answer is:
baloney
'rubbish' is the definition.
(I've seen this in another clue)
'solitary in tree?' is the wordplay.
'solitary' becomes 'lone' (synonyms).
'in' is an insertion indicator.
'tree?' becomes 'bay' (bay is a kind of tree).
'lone' going inside 'bay' is 'BALONEY'.
(Other definitions for baloney that I've seen before include "foolish talk!" , "rhubarb" , "Tosh" , "deceptive talk" , "Tripe < , smoked sausage" .)