Aunt blethered freely, revealing secret (5-3-5)
I believe the answer is:
under-the-table
'secret' is the definition.
('under-the-table' can be a synonym of 'secret')
'aunt blethered freely' is the subsidiary indication.
'freely' is an anagram indicator.
'aunt'+'blethered'='auntblethered'
'auntblethered' anagrammed gives 'underthetable'.
'revealing' is the link.
(Other definitions for under-the-table that I've seen before include "Not above board" , "Illicit; hopelessly drunk" , "Transacted in secret -- sozzled" , "Hush-hush" .)