Method used to measure weight, one hears (3)
I believe the answer is:
way
'method' is the definition.
(way of doing something)
'weight one hears' is the wordplay.
'weight' becomes 'w' (short for weight).
'one' becomes 'I' (Roman numeral).
'hears' indicates a 'sounds like' (homophone) clue (I've seen 'heard of' mean this).
'i' sounds like 'ay'.
'w'+'ay'='WAY'
'used to measure' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for way that I've seen before include "Route; direction" , "Method; route" , "Manner; road" , "'Manner, mode (3)'" , "Path" .)