Rattle never developed by one in France (7)
I believe the answer is:
unnerve
'rattle' is the definition.
(thesaurus)
'never developed by one in france' is the wordplay.
'developed' is an anagram indicator.
'by' means one lot of letters go next to another.
'one in france' becomes 'un'.
'never' is an anagram of 'nerve'.
'nerve' after 'un' is 'UNNERVE'.
(Other definitions for unnerve that I've seen before include "Disheartened" , "Discourage - demoralise" , "Deprive of confidence" , "Cause to lose courage - even run away" , "Psych out" .)