Right to enter half-open bay, say (6)
I believe the answer is:
entree
'right to enter' is the definition.
(I've seen this in another clue)
'half-open bay say' is the wordplay.
'half' means to take half the letters.
'bay say' becomes 'tree' (bay is a kind of tree).
'open' cut in half is 'en'.
'en'+'tree'='ENTREE'
(Other definitions for entree that I've seen before include "Right of access; course" , "Privilege of admission, or food dish" , "Means of access - course" , "Main course of a meal" , "Dish allowing one to go in?" .)