Not worth considering one Frenchman has potential (10)
I believe the answer is:
impossible
'not worth considering' is the definition.
I can't judge whether this definition defines the answer.
'one frenchman has potential' is the wordplay.
'one' becomes 'i' (Roman numeral).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'has' is a charade indicator (letters next to each other).
'potential' becomes 'possible' ('possible' can be a synonym of 'potential'**).
'i'+'m'+'possible'='IMPOSSIBLE'
Can you help me to learn more?
(Other definitions for impossible that I've seen before include "Hopeless" , "See 14 down" , "some kind of mission" , "Not able to exist" , "See 10" .)