Guide for better parking in line across street (7)
I believe the answer is:
tipster
'guide for better' is the definition.
(guide for someone making bets)
'parking in line across street' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'in' indicates putting letters inside.
'line' becomes 'tier' (tier is a kind of line).
'across' indicates putting letters inside.
'street' becomes 'st' (abbreviation).
'p' going inside 'tier' is 'tiper'.
'tiper' placed around 'st' is 'TIPSTER'.
(Other definitions for tipster that I've seen before include "Supposed expert on horses" , "One providing betting hints" , "better counsellor" , "I have a horse" , "Giver of betting advice" .)