A French monsieur of French team turned straight (7)
I believe the answer is:
unmixed
'straight' is the definition.
(thesaurus)
'a french monsieur of french team turned' is the wordplay.
'a french' becomes 'un' ('a' in French).
'monsieur' becomes 'm'.
'of french' becomes 'de'.
'team' becomes 'xi' (eleven, a football team).
'turned' is a reversal indicator.
'de'+'xi'='dexi'
'dexi' backwards is 'ixed'.
'un'+'m'+'ixed'='UNMIXED'
(Other definitions for unmixed that I've seen before include "Separate" , "Pure" .)