Gemstone, in the morning, they mislaid on street (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'morning they mislaid on street' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'mislaid' is an anagram indicator.
'on' indicates putting letters inside (as in clothing 'on' a person).
'street' becomes 'st' (abbreviation).
'they' is an anagram of 'ethy'.
'am' placed within 'ethy' is 'amethy'.
'amethy'+'st'='AMETHYST'
'in the' is the link.
(Other definitions for amethyst that I've seen before include "A Middle East language" , "rock" , "Something precious" , "Variety of quartz" , "One showing sparkling facets" .)