Opening Sabbath set (4)
I believe the answer is:
slot
'opening' is the definition.
(I've seen this before)
'sabbath set' is the wordplay.
'sabbath' becomes 's'.
'set' becomes 'lot' ('lot' can be a synonym of 'set'**).
's'+'lot'='SLOT'
(Other definitions for slot that I've seen before include "Space for coin or programme" , "Opening (for coins)" , "Long narrow aperture" , "Lots (anag.)" , "Opening for coin or TV programme" .)