Oak nut (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
acorn
'oak nut' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for acorn that I've seen before include "oak's fruit" , "Tree seed" , "mast" , "From which a "tall oak" may grow" , "old computer?" .)