Least good street behind terrace knocked over (5)
I believe the answer is:
worst
'least good' is the definition.
(most bad)
'street behind terrace knocked over' is the wordplay.
'street' becomes 'st' (abbreviation).
'behind' means one lot of letters go next to another.
'terrace' becomes 'row'.
'knocked over' is a reversal indicator.
'row' in reverse letter order is 'wor'.
'st' put after 'wor' is 'WORST'.
(Other definitions for worst that I've seen before include "Most inferior or evil" , "Most unpleasant" , "Most badly" , "the pits" , "Of the poorest quality" .)