Creatures initially face an unusual nocturnal arrangement (5)
I believe the answer is:
fauna
'creatures' is the definition.
(I've seen this in another clue)
'initially face an unusual nocturnal arrangement' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'face an unusual nocturnal arrangement' is 'FAUNA'.
(Other definitions for fauna that I've seen before include "The animals found in a particular area" , "Wild animal life of an era" , "Animal life of an area" , "Animal life of given region" , "Local animal life" .)