Fancy peccadillo finally committed away from home (6)
I believe the answer is:
notion
'fancy' is the definition.
(both can mean an idea or whim)
'peccadillo finally committed away from home' is the wordplay.
'finally' indicates one should take the final letters.
'committed' is an insertion indicator (inserted letters are contained or committed).
'away from home' becomes 'not in' ('she's not in' means 'she's not home').
The last letter of 'peccadillo' is 'o'.
'o' put into 'notin' is 'NOTION'.
(Other definitions for notion that I've seen before include "Whim or fancy" , "desire" , "Idea or whim" , "clue" , "Idea or fancy" .)