Scientific unit needing old trees destroyed (fifty to go) (7)
I believe the answer is:
oersted
'scientific unit' is the definition.
(I know that oersted is scientific)
I don't understand how the remainder of the clue works.
(Other definitions for oersted that I've seen before include "Physics unit of magnetic field strength" , "Physical unit" , "Ted Rose (anag) - Danish physicist who discovered electromagnetism" .)