Poorest quality terrace put up on street (5)
I believe the answer is:
worst
'poorest quality' is the definition.
'terrace put up on street' is the wordplay.
'terrace' becomes 'row'.
'put up' says the letters should be written in reverse.
'on street' becomes 'st' (I've seen this in other clues).
'row' backwards is 'wor'.
'wor'+'st'='WORST'
(Other definitions for worst that I've seen before include "the pits" , "Most severe" , "Best; least good" , "Direst" , "Least well" .)