Precious stones are measured in article in vehicle (5)
I believe the answer is:
carat
'precious stones are measured' is the definition.
I can't tell whether this defines the answer.
'article in vehicle' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'in' is an insertion indicator.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'a' going within 'cart' is 'CARAT'.
'in' is the link.
Can you help me to learn more?
(Other definitions for carat that I've seen before include "A unit of weight for precious stones" , "Measure of purity of gold" , "Measure for diamonds or gold" , "Unit of gem measurement" , "Unit of weight for gems or purity of gold" .)