Quickly shifting the blame — in vain (4)
I believe the answer is:
idly
'in vain' is the definition.
(I've seen this before)
'quickly shifting the blame' is the wordplay.
'quickly' becomes 'rapidly' (synonyms).
'shifting' is a deletion indicator.
'the blame' becomes 'rap' (as in taking the rap for something).
'rapidly' with 'rap' taken out is 'IDLY'.
(Other definitions for idly that I've seen before include "Lazily, without due attention" , "How one stands by?" , "in a careless manner" , "in a way vain" , "In a lazy fashion" .)