Poem in French is popular, widening appeal (7)
I believe the answer is:
sestina
'poem' is the definition.
(thesaurus)
'french is popular widening appeal' is the wordplay.
'french is' becomes 'est' ('is' in the French language).
'popular' becomes 'in'.
'widening' means one lot of letters goes inside another (inserted letters make the original word wider).
'appeal' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'est'+'in'='estin'
'estin' placed within 'sa' is 'SESTINA'.
'in' acts as a link.
(Other definitions for sestina that I've seen before include "six-line stanza" , "Old verse form" , "Highly structured poem of 39 lines" , "Form of poem" , "poetic structure" .)