Tree-dwarfing art (6)
I believe the answer is:
bonsai
'tree-dwarfing art' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bonsai that I've seen before include "Art of dwarfing shrubs" , "Little plants" , "Mini-tree growing" , "Dwarfed ornamental tree or shrub" , "The art of growing ornamental dwarfed plants" .)