### Clue: Sis neglected to re-emphasise arrangement of a transient nature (8)
### Answer: EPHEMERA 
 ### Explanation: Definition: of a transient nature
 "sis" **deleted from**(neglected to) **anagram of**(… arrangement) "re-emphasise".
Defn: It leads to an adjective, unless one considers that "things" is implicitly in front, to make it a noun, as the answer is.