Crossing street, certainly not horrible (5)
I believe the answer is:
nasty
'horrible' is the definition.
(I've seen this before)
'crossing street certainly not' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'street' becomes 'st' (abbreviation).
'certainly not' becomes 'nay' (archaic form of 'no').
'st' inserted within 'nay' is 'NASTY'.
(Other definitions for nasty that I've seen before include "Horrid, repugnant" , "Repulsive" , "Offensive or malicious" , "Mean" , "Evil" .)