Slow-growing trees (4)
I believe the answer is:
oaks
'slow-growing trees' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oaks that I've seen before include "Acorn trees" , "material for shipbuilding" , "Trees of the genus Quercus" , "Epsom Classic race" , "Hardwood trees" .)