A hundred Bob lost on The French Stone (6)
I believe the answer is:
cobble
'stone' is the definition.
(I know that cobble is a type of paving stone)
'a hundred bob lost on the french' is the wordplay.
'a hundred' becomes 'c' (Roman numeral).
'lost' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'the french' becomes 'le' ('the' in French).
'bob' with letters rearranged gives 'obb'.
'c' inserted into 'obb' is 'cobb'.
'cobb'+'le'='COBBLE'
(Other definitions for cobble that I've seen before include "Mend shoes with a paving stone?" , "Road-surfacing stone" , "piece of paving" , "Rounded stone once commonly used for paving" , "Mend shoes - round stone" .)