Cavities in adult supported by detailed examination (5)
I believe the answer is:
atria
'cavities' is the definition.
(atrium is a kind of cavity)
'adult supported by detailed examination' is the wordplay.
'adult' becomes 'a' (abbreviation).
'supported by' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'examination' becomes 'trial' (I've seen this in another clue).
'trial' with its final letter taken off is 'tria'.
'a'+'tria'='ATRIA'
'in' acts as a link.
(Other definitions for atria that I've seen before include "Public spaces with galleries" , "Apartments" , "Open-roofed entrance halls (Roman) - glass-roofed central halls (modern)" , "Lobbies" , "Upper chambers of the heart" .)