Infant born in a tree (4)
I believe the answer is:
baby
'infant' is the definition.
(I know that infant can be written as baby)
'born in a tree' is the wordplay.
'born' becomes 'b' (abbreviation next to the year of someone's birth).
'in' indicates putting letters inside.
'a tree' becomes 'bay' (bay is a kind of tree).
'b' going inside 'bay' is 'BABY'.
(Other definitions for baby that I've seen before include "Very small (version)" , "Person born just after WWII" , "Love" , "New-born child" , "Mollycoddle" .)