Most fashionable tendency that's on the way (9)
I believe the answer is:
trendiest
'most fashionable' is the definition.
('trendy' can be similar in meaning to 'fashionable')
'tendency that's on the way' is the wordplay.
'tendency' becomes 'trend' (trend is a kind of tendency).
'that's' becomes 'ie'.
'on' is a charade indicator (letters next to each other).
'the way' becomes 'st' (abbreviation for street).
'trend'+'ie'+'st'='TRENDIEST'
(Other definitions for trendiest that I've seen before include "Most cool!" , "cooler than the rest" , "Most popular" .)