Tree by vacant lavatory is getting old (7)
I believe the answer is:
elderly
'old' is the definition.
('elderly' can be similar in meaning to 'old')
'tree by vacant lavatory is getting' is the wordplay.
'tree by' becomes 'elder' (elder is a kind of tree. I am not sure about the 'by' bit.**).
'vacant' means to remove the middle letters.
'is getting' says to put letters next to each other.
'lavatory' with its centre removed is 'ly'.
'elder'+'ly'='ELDERLY'
(Other definitions for elderly that I've seen before include "Not young" , "Fairly old, past middle age" , "Getting on in years" , "over the hill" , "Somewhat old" .)