Length of some tress? (5)
I believe the answer is:
metre
'length' is the definition.
(I've seen this before)
'of some tress?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'METRE' is hidden within 'some tress'.
(Other definitions for metre that I've seen before include "Beat pattern (mus.)" , "Approx 39 inches" , "Characteristic of feet" , "Basic unit of length" , "Yard is smaller than this" .)