First-born tree (5)
I believe the answer is:
elder
I believe this is a double definition.
'first-born' is the first definition.
(thesaurus)
'tree' is the second definition.
(elder is a kind of tree)
(Other definitions for elder that I've seen before include "Character of church" , "Shrub with white flowers and berrylike fruit" , "one with roots" , "Tree; of greater age" , "Senior member - plant" .)