Start docking vessel after nightfall? (4)
I believe the answer is:
dark
'nightfall?' is the definition.
(I've seen this in another clue)
'start docking vessel after' is the wordplay.
'start' indicates taking the first letters.
'vessel' becomes 'ark' (ark is a kind of vessel).
'after' is a charade indicator (letters next to each other).
The initial letter of 'docking' is 'd'.
'd'+'ark'='DARK'
(Other definitions for dark that I've seen before include "Black, perhaps - night" , "With little or no light" , "Absence of light" , "Closed as theatre" , "Unlit" .)