I'm surprised seeing Frenchman with salad, as a rule (4,3)
I believe the answer is:
ohm's law
'a rule' is the definition.
(I have seen 'Current rule' mean 'ohms law' so perhaps 'rule' could also mean 'ohms law')
'i'm surprised seeing frenchman with salad' is the wordplay.
'i'm surprised' becomes 'oh' (exclamation).
'seeing frenchman' becomes 'm' (abbreviation for Monsieur).
'with' is a charade indicator (letters next to each other).
'salad' becomes 'slaw' (slaw is a kind of salad).
'oh'+'m'+'slaw'='OHMS LAW'
'as' is the link.
(Another definition for ohm's law that I've seen is " V = IR".)