The Ruin of Hitler, a parting novel by Stephen (3,4)

'novel by stephen' is the definition.
(Stephen Fry novel)

'the ruin of hitler a parting' is the wordplay.
'the ruin of' is an anagram indicator (I've seen 'the ruins of' mean this).
'parting' indicates putting letters inside (one word creates a gap in ie parts another word).
'hitler' with letters rearranged gives 'thelir'.
'thelir' placed around 'a' is 'THE LIAR'.