Opening or almost not opening (5)
I believe the answer is:
early
'opening' is the definition.
(I've seen this before)
'almost not opening' is the wordplay.
'almost' becomes 'nearly' (synonyms).
'not opening' means to remove the first letter (the opening letter is removed).
'nearly' with its initial letter taken away is 'EARLY'.
'or' acts as a link.
(Other definitions for early that I've seen before include "Premature" , "New-season" , "Post this for Christmas?" , "Advanced" , "sooner than expected" .)