Taking forever to cross street might be this? On the contrary! (6)
I believe the answer is:
sagest
'be this? on the contrary' is the definition.
The definition suggests a singular noun but the answer is not.
'taking forever to cross street' is the wordplay.
'taking' means one lot of letters goes inside another.
'forever' becomes 'ages' (I've seen this in another clue).
'to cross street' becomes 'st' (I can't justify this - if you can you should believe this answer much more).
'ages' inserted inside 'st' is 'SAGEST'.
'might' acts as a link.
Can you help me to learn more?
(Other definitions for sagest that I've seen before include "Extremely clever" , "Wisest" , "Most discerning" , "Gas set (anag.)" , "Most wise and prudent" .)