Over the hill, yet initially below tree line (7)
I believe the answer is:
elderly
'over the hill' is the definition.
(thesaurus)
'yet initially below tree line' is the wordplay.
'initially' indicates taking the first letters.
'below' says to put letters next to each other.
'tree' becomes 'elder' (elder is a kind of tree**).
'line' becomes 'l'.
The initial letter of 'yet' is 'y'.
'elder'+'l'='elderl'
'y' put after 'elderl' is 'ELDERLY'.
(Other definitions for elderly that I've seen before include "Long in the tooth" , "like a rich attorney's ugly daughter" , "Aged" , "Somewhat old" , "Getting on a bit" .)