In agreement for a French crash diet (6)
I believe the answer is:
united
'in agreement' is the definition.
('united' can be similar in meaning to 'in agreement')
'a french crash diet' is the wordplay.
'a french' becomes 'un' ('a' in French**).
'crash' indicates an anagram.
'diet' is an anagram of 'ited'.
'un'+'ited'='UNITED'
'for' is the link.
(Other definitions for united that I've seen before include "cooperative" , "Mark -, Russian-born US abstract painter" , "Joined into a single entity" , "See 10" , "Joined together like Manchester soccer team" .)