Solitary in tree? Rubbish! (7)
I believe the answer is:
'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" .)