Marries a French countess, at last admitting appeal (6)
I believe the answer is:
unites
'marries' is the definition.
(I've seen this before)
'a french countess at last admitting appeal' is the wordplay.
'a french' becomes 'une' ('a' in French).
'at last' suggests the final letters.
'admitting' indicates putting letters inside (admit can mean to welcome in or accept).
'appeal' becomes 'it' ('it' can mean something attractive or appealing).
The final letter of 'countess' is 's'.
'une'+'s'='unes'
'unes' enclosing 'it' is 'UNITES'.
(Other definitions for unites that I've seen before include "Joins together as one" , "Combines" , "Comes together as one" , "Becomes one" , "Comes together for a common purpose" .)