Length of a lime tree! (5)
I believe the answer is:
metre
'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 "Poetic rhythm" , "Basic unit of length" , "Approximately 39 inches" , "Rhythmic arrangement of syllables in verse" , "Distance" .)