Stone Age leader with me on your street (8)
I believe the answer is:
amethyst
'stone' is the definition.
(I've seen this before)
'age leader with me on your street' is the wordplay.
'leader' says to take the initial letters.
'with' means one lot of letters go next to another.
'on' means one lot of letters go next to another.
'your' becomes 'thy' ('thy' historically meant 'your').
'street' becomes 'st' (abbreviation).
The first letter of 'age' is 'a'.
'a'+'me'+'thy'+'st'='AMETHYST'
(Other definitions for amethyst that I've seen before include "Something precious" , "Purple gem" , "One showing sparkling facets" , "what's in ring" , "Violet gemstone" .)