Old copper in standard valve (8)
I believe the answer is:
stopcock
'valve' is the definition.
(thesaurus)
'old copper in standard' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'copper' becomes 'pc'.
'in' means one lot of letters goes inside another.
'standard' becomes 'stock' ('stock' can be a synonym of 'standard'**).
'o'+'pc'='opc'
'opc' put inside 'stock' is 'STOPCOCK'.
(Other definitions for stopcock that I've seen before include "water regulator" , "Halt rooster with pipe valve" , "it may interrupt the flow" , "Control of flow in pipe" , "Valve in water supply" .)