A large bay, or recess (6)
I believe the answer is:
alcove
'recess' is the definition.
(I know that alcove is a type of recess)
'a large bay' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'bay' becomes 'cove' (I've seen this in another clue).
'a'+'l'+'cove'='ALCOVE'
'or' acts as a link.
(Other definitions for alcove that I've seen before include "corner" , "bay" , "Recessed part of a room" , "private part?" , "Small opening - a clove (anag)" .)