Joined in a French form of diet (6)
I believe the answer is:
united
'joined' is the definition.
(I've seen this before)
'a french form of diet' is the wordplay.
'a french' becomes 'un' ('a' in French**).
'form of' is an anagram indicator (the letters need to be in another 'form').
'diet' anagrammed gives 'ited'.
'un'+'ited'='UNITED'
'in' acts as a link.
(Other definitions for united that I've seen before include "Joined together, allied" , "Amalgamated" , "Manchester _____, current Premiership champions with..." , "collective" , "Whole" .)