An opening possibly lost (4)
I believe the answer is:
slot
'an opening' is the definition.
(I know that slot is a type of slit)
'possibly lost' is the wordplay.
'possibly' is an anagram indicator.
'lost' anagrammed gives 'SLOT'.
(Other definitions for slot that I've seen before include "This gaming machine operates by insertion of coin" , "Opening for coin or TV programme" , "Scheduled time - hierarchical position" , "Long narrow opening" , "Long narrow slit" .)