Evening Star's first open when empty (7)
I believe the answer is:
sundown
'evening' is the definition.
(the sun goes down in the evening)
'star's first open when empty' is the wordplay.
'star's first' becomes 's' (1st letter of 'star').
'open' becomes 'undo' (eg opening a bottle).
'empty' suggests removing the centre.
'when' with its centre taken out is 'wn'.
's'+'undo'+'wn'='SUNDOWN'
(Other definitions for sundown that I've seen before include "one time in the evening" , "When a star declines" , "when the day is done" , "Part of evening" , "End of the day" .)