CGS unit formerly included in dictionary (7)
I believe the answer is:
oersted
'cgs unit' is the definition.
(unit in the centimetre-gram-second system)
'formerly included in dictionary' is the wordplay.
'formerly' becomes 'erst' (archaic term for formerly or long ago).
'included in' means one lot of letters goes inside another.
'dictionary' becomes 'oed' (Oxford English Dictionary).
'erst' placed into 'oed' is 'OERSTED'.
(Other definitions for oersted that I've seen before include "Ted Rose (anag) - Danish physicist who discovered electromagnetism" , "Physical unit" , "Physics unit of magnetic field strength" .)