Line written by individual in tree is nonsense (7)
I believe the answer is:
baloney
'is nonsense' is the definition.
(I've seen this before)
'line written by individual in tree' is the wordplay.
'line' becomes 'l'.
'written by' means one lot of letters go next to another.
'individual' becomes 'one' (an individual is one person).
'in' indicates putting letters inside.
'tree' becomes 'bay' (bay is a kind of tree).
'l'+'one'='lone'
'lone' inserted inside 'bay' is 'BALONEY'.
(Other definitions for baloney that I've seen before include "deceptive talk" , "Nonsense; lean yob (anag)" , "Cobblers" , "Tosh" , "rhubarb" .)