Least wobbly magnetic unit (5)
I believe the answer is:
tesla
'magnetic unit' is the definition.
(I've seen this before)
'least wobbly' is the wordplay.
'wobbly' indicates an anagram.
'least' is an anagram of 'TESLA'.
(Other definitions for tesla that I've seen before include "SI unit; steal (anag.)" , "American scientist" , "electrical unit" , "Physicist" , "Inventive sort" .)