Hear about 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)
'hear about old' is the wordplay.
'hear' becomes 'try' ('try' can be a synonym of 'hear').
'about' is an insertion indicator.
'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 "Precious stone weight system" , "Priam's city" , "System of weights for measuring precious metals and gemstones" , "City besieged by Greeks to rescue Helen" , "City besieged by the Greeks - system of gem weight measurement" .)