Aunt ruthlessly conceals lie (7)
I believe the answer is:
untruth
'lie' is the definition.
(thesaurus)
'aunt ruthlessly conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'UNTRUTH' is hidden in the letters of 'aunt ruthlessly'.
(Other definitions for untruth that I've seen before include "product of Orwell's ministry?" , "story" , "Falsehood or lie" , "A lie or false statement" , "A genuinely deniable" .)