Main stems of trees (6)
I believe the answer is:
trunks
'main stems of trees' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trunks that I've seen before include "Parts of trees and elephants" , "Men's swimsuit" , "Main stems of trees" , "Men's swimsuits on an elephants?" , "luggage" .)