Endless parking in streets regulated (10)
I believe the answer is:
persistent
'endless' is the definition.
(I've seen this before)
'parking in streets regulated' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'regulated' indicates an anagram.
'in'+'streets'='instreets'
'instreets' anagrammed gives 'ersistent'.
'p'+'ersistent'='PERSISTENT'
(Other definitions for persistent that I've seen before include "Unrelenting - prettiness (anag)" , "Prolonged; nagging" , "Lasting, prolonged" , "Relentless" , "fixed" .)