Connoisseurs, one hundred with terrible congestion (11)
I believe the answer is:
cognoscenti
'connoisseurs' is the definition.
(I've seen this before)
'one hundred with terrible congestion' is the wordplay.
'one hundred' becomes 'c' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'terrible' indicates anagramming the letters.
'congestion' is an anagram of 'ognoscenti'.
'c'+'ognoscenti'='COGNOSCENTI'
(Other definitions for cognoscenti that I've seen before include "Well-informed people" , "Informed experts" , "Subject experts" , "discerning experts?" , "Sophisticates" .)