Length of a lime tree! (5)
I believe the answer is:
'length' is the definition.
(I've seen this before)
'of a lime tree' is the wordplay.
'of' says the answer is hidden in the clue.
'METRE' can be found hidden inside 'a lime tree'.
(Other definitions for metre that I've seen before include "determined by scanning", "Unit of length in metric system", "Beat pattern (mus.)", "Characteristic of feet", "100cm".)