Attempt to keep old system of weights (4)
I believe the answer is:
troy
'system of weights' is the definition.
(troy is a kind of system of weights)
'attempt to keep old' is the wordplay.
'attempt' becomes 'try' (I've seen this before).
'to keep' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'try' placed around 'o' is 'TROY'.
(Other definitions for troy that I've seen before include "Priam's city" , "System of weights; ancient city in Turkey" , "System of weights introduced by French" , "part of Asia Minor" , "Hardy sergeant" .)