The best in a very posh street (6)
I believe the answer is:
finest
'the best' is the definition.
(the most excellent or fine)
'very posh street' is the wordplay.
'very posh' becomes 'fine' (I can't justify this - if you can you should give a lot more credence to this answer).
'street' becomes 'st' (abbreviation).
'fine'+'st'='FINEST'
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for finest that I've seen before include "Of the highest quality" , "Most delicate or topmost" , "The very best or most gossamer" , "Of the highest grade" , "Most exquisite" .)