Ted's interchange is most fashionable (9)
I believe the answer is:
trendiest
'is most fashionable' is the definition.
('trendy' can be similar in meaning to 'fashionable')
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for trendiest that I've seen before include "Most cool!" , "Most popular" , "cooler than the rest" .)