Crossword Genius

Scientific unit needing old trees destroyed (fifty to go) (7)

Ross

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" .)

I've seen this clue in The Times.
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