Hollow space (6)
(Other definitions for recess that I've seen before include "Short break" , "Niche, alcove" , "secluded spot" , "When Parliament is not sitting" , "Alcove for remission of business" .)
'hollow space' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cavity that I've seen before include "Hollow place" , "Hole, crater" , "Hole, gap" , "Empty space" , "Internal void" .)