Unimportant details of short journey involving four assassins originally (6)
I believe the answer is:
trivia
'unimportant details' is the definition.
(I've seen this before)
'short journey involving four assassins originally' is the wordplay.
'short journey' becomes 'tri' (I can't explain this - if you can you should believe this answer much more).
'involving' means one lot of letters goes inside another.
'four' becomes 'iv' (roman numeral).
'originally' suggests taking the first letters.
The first letter of 'assassins' is 'a'.
'tri' enclosing 'iv' is 'trivi'.
'trivi'+'a'='TRIVIA'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for trivia that I've seen before include "Minor details" , "Unimportant or inconsequential things" , "Something of little importance" , "petty considerations" , "minor points of interest?" .)