The French world, first and last, is vulgar (4)
I believe the answer is:
lewd
'vulgar' is the definition.
(thesaurus)
'the french world first and last' is the wordplay.
'the french' becomes 'le'.
'first' indicates taking the first letters.
'last' suggests the final letters.
The first letter of 'world' is 'w'.
The final letter of 'and' is 'd'.
'le'+'w'+'d'='LEWD'
'is' is the link.
(Other definitions for lewd that I've seen before include "Salacious" , "blue" , "Obscene, indecent" , "Bawdy" , "Indecent, licentious" .)