In a shop a lustrous jewel is seen (4)
I believe the answer is:
opal
'jewel is seen' is the definition.
I can't tell whether this defines the answer.
'in a shop a lustrous' is the wordplay.
'in' says the answer is hidden in the clue.
'OPAL' is hidden within 'a shop a lustrous'.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "National gemstone of Australia" , "Form of hydrated silica, often used as a gemstone" , "Semi-precious stone of milky or bluish colour" , "Type of gem" , "Gem-stone with a milky quality" .)