Turn amber and leave bar (7)
I believe the answer is:
embargo
'bar' is the definition.
(thesaurus)
'turn amber and leave' is the wordplay.
'turn' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'leave' becomes 'go'.
'amber' anagrammed gives 'embar'.
'embar'+'go'='EMBARGO'
(Other definitions for embargo that I've seen before include "Government order imposing a trade barrier" , "Official order forbidding something" , "Admiralty order" , "Prohibit" , "Trade barrier or publication ban" .)