Succinct poem needs new beginning (5)
I believe the answer is:
terse
'succinct' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
(Other definitions for terse that I've seen before include "Reset to be pithy" , "Brief and to the point" , "Trees (anag.)" , "Abrupt, concise" , "Succinct about trees" .)