It's used on watch (6-4)
'it's used' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for secondhand that I've seen before include "feature on a watch" , "Used" .)
(Other definitions for second-hand that I've seen before include "Previously owned - part of watch" , "Not original" , "Used" , "Not new - part of watch!" .)