Improve street that's law-abiding (6)
I believe the answer is:
honest
'law-abiding' is the definition.
(I've seen this before)
'improve street' is the wordplay.
'improve' becomes 'hone' (honing is a kind of improving).
'street' becomes 'st' (abbreviation).
'hone'+'st'='HONEST'
'that's' acts as a link.
(Other definitions for honest that I've seen before include "Impartial intermediary" , "Without dissimulation or guile" , "Objective mediator" , "Straight up" , "Upright and straightforward" .)