Red toes form a magnetic unit (7)
I believe the answer is:
oersted
'a magnetic unit' is the definition.
(I've seen this in another clue)
'red toes form' is the wordplay.
'form' indicates an anagram (another form of the letters).
'red'+'toes'='redtoes'
'redtoes' anagrammed gives 'OERSTED'.
(Other definitions for oersted that I've seen before include "Physics unit of magnetic field strength" , "Ted Rose (anag) - Danish physicist who discovered electromagnetism" , "Physical unit" .)