The tree by the way is elder (5)
I believe the answer is:
first
'is elder' is the definition.
(thesaurus)
'the tree by the way' is the wordplay.
'the tree' becomes 'fir' (fir is a kind of tree).
'by' means one lot of letters go next to another.
'the way' becomes 'st'.
'fir'+'st'='FIRST'
(Other definitions for first that I've seen before include "The one that precedes all others, in time say" , "Preliminary" , "Coming before all others" , "Top degree class" , "Original" .)