Fruit of the oak tree (5)
(Other definitions for acorns that I've seen before include "Seeds of the oak" , "From which 23 down grow" , "From which great oak trees grow" , "wild fruit" , "Potential oaks?" .)
'fruit of the oak tree' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for acorn that I've seen before include "From which a tall tree may grow" , "Dirt on a liquid surface" , "form of seeding" , "Model in stone" , "From which a "tall oak" may grow" .)