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' indicates putting letters inside.
'tree?' becomes 'bay' (bay tree is a kind of tree).
'lone' inserted inside 'bay' is 'BALONEY'.
(Other definitions for baloney that I've seen before include "foolish talk!" , "rhubarb" , "Tosh" , "deceptive talk" , "Tripe < , smoked sausage" .)