In conclusion, sample exterior (6)
I believe the answer is:
trendy
'in' is the definition.
('in' can mean trendy or fashionable)
'conclusion sample exterior' is the wordplay.
'conclusion' becomes 'end' (something's conclusion is its end).
'sample' becomes 'try' (synonyms).
'exterior' indicates putting letters inside (some letters go on the outside of others).
'end' put within 'try' is 'TRENDY'.
(Other definitions for trendy that I've seen before include "With-it" , "Informally, following the fashions" , "'Hip, fashionable (6)'" , "In vogue" , "Informally in the latest fashion" .)