Second shed not open (4)
I believe the answer is:
shut
'not open' is the definition.
('shut' is the opposite to 'open')
'second shed' is the wordplay.
'second' becomes 's' (common abbreviation).
'shed' becomes 'hut' (I've seen this in another clue).
's'+'hut'='SHUT'
(Other definitions for shut that I've seen before include "Push to" , "Slam" , "Draw to" , "Close" , "Out of business" .)