As before dark, in twilit thicket observation starts (5)
I believe the answer is:
ditto
'as before' is the definition.
(I've seen this before)
'dark in twilit thicket observation starts' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'dark in twilit thicket observation' is 'DITTO'.
(Other definitions for ditto that I've seen before include "Mark used to indicate the word above it should be repeated" , "Two dots say same again" , "Repeat the word(s) above" , "Copy" , "Do alternative" .)