Evening Standard's first open when free (7)
I believe the answer is:
sundown
'evening' is the definition.
(I've seen this before)
'standard's first open when free' is the wordplay.
'standard's first' becomes 's' (wst letter of 'standard').
'open' becomes 'undo' (I've seen this before).
'when free' becomes 'wn' (I am not sure about this - if you are sure you should believe this answer much more).
's'+'undo'+'wn'='SUNDOWN'
Can you help me to learn more?
(Other definitions for sundown that I've seen before include "when night life would start?" , "End of the day" , "Setting time" , "Event seen in evening" , "Feature of evening" .)