Firstborn in tree (5)
I believe the answer is:
elder
This clue might be a double definition.
'firstborn' is the first definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'tree' is the second definition.
(elder is a kind of tree)
'in' acts as a link.
Can you help me to learn more?
(Other definitions for elder that I've seen before include "senior church member" , "Tree - having lived longer" , "Ancestor" , "Senior member - plant" , "Church official; tree" .)