Such a tree no elm could be (5)
I believe the answer is:
lemon
'such a tree' is the definition.
(lemon is a kind of tree)
'no elm could be' is the wordplay.
'could be' indicates anagramming the letters.
'no'+'elm'='noelm'
'noelm' is an anagram of 'LEMON'.
(Other definitions for lemon that I've seen before include "very different from peach?" , "A fruit; a worthless item" , "Fruit (which is proverbially "the answer")" , "Pulsate" , "failure" .)