Oak-to-be (5)
I believe the answer is:
acorn
'oak-to-be' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acorn that I've seen before include "Baby oak?" , "Nut of oak tree" , "oak's fruit" , "From which a "tall oak" may grow" , "Potential oak" .)