Mistake in Faraday's first law (4)
I believe the answer is:
flaw
'mistake' is the definition.
(thesaurus)
'faraday's first law' is the wordplay.
'faraday's first' becomes 'f' (1st letter of 'faraday').
'f'+'law'='FLAW'
'in' is the link.
(Other definitions for flaw that I've seen before include "Imperfection or blemish" , "(Make) defect(ive)" , "sudden rush" , "Weakness" , "failing" .)