Maintaining speed, a van in York street is purple (8)
I believe the answer is:
amethyst
'is purple' is the definition.
(thesaurus)
'maintaining speed a van in york street' is the wordplay.
'maintaining' means one lot of letters goes inside another.
'speed' becomes 'meth' (synonyms).
'van in york street' becomes 'yst' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'yst'='ayst'
'meth' placed inside 'ayst' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Purple quartz, used as a gemstone" , "Purplish gemstone" , "Transparent bluish quartz" , "One showing sparkling facets" , "Bluish-violet 3" .)