Trees may be pithy (5)
I believe the answer is:
terse
'pithy' is the definition.
(I've seen this before)
'trees may be' is the wordplay.
'may be' indicates anagramming the letters.
'trees' with letters rearranged gives 'TERSE'.
(Other definitions for terse that I've seen before include "Of speech, short and pithy" , "Brief and to the point" , "Briefly expressed" , "Crisply brief" , "Curtly brief" .)