The wrong thing to drop at dusk (9)
I believe the answer is:
nightfall
'dusk' is the definition.
(I know that dusk can be written as nightfall)
'the wrong thing to drop' is the wordplay.
'the wrong' is an anagram indicator.
'to drop' becomes 'fall' (synonyms).
'thing' with letters rearranged gives 'night'.
'night'+'fall'='NIGHTFALL'
'at' is the link.
(Other definitions for nightfall that I've seen before include "The onset of darkness" , "Dusk" , "start of the dark time" , "Arrival of darkness" , "when visibility reduces" .)