Most fashionable, but comes to an end in river (9)
I believe the answer is:
trendiest
'most fashionable' is the definition.
('trendy' can be similar in meaning to 'fashionable')
'comes to an end in river' is the wordplay.
'comes to an end' becomes 'dies' (I've seen this before).
'in' means one lot of letters goes inside another.
'river' becomes 'trent' (I've seen this before**).
'dies' placed inside 'trent' is 'TRENDIEST'.
'but' is the link.
(Other definitions for trendiest that I've seen before include "Most popular" , "Most cool!" , "cooler than the rest" .)