Fashionable dude disheartened after mission in Texas (1,2,4)
I believe the answer is:
a la mode
'fashionable' is the definition.
(a la mode can mean trendy or fashionable)
'dude disheartened after mission in texas' is the wordplay.
'disheartened' means to remove the middle letters (the middle or heart is removed).
'after' means one lot of letters go next to another.
'mission in texas' becomes 'alamo' (I've seen this before).
'dude' with its middle removed is 'de'.
'de' after 'alamo' is 'A-LA-MODE'.
(Other definitions for a la mode that I've seen before include "With ice cream on top or by the side" , "In the fashion - in France" , "Fashionable (French)" , "Do a meal in French style (1, 2, 4)" , "Fashionable -- served with ice cream" .)