Vanity cases? (4)
I believe the answer is:
egos
'vanity cases?' is the definition.
(I've seen this in another clue)
'vanity cases?' is the wordplay.
'vanity' becomes 'ego' (I've seen this in another clue).
'cases?' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ego'+'s'='EGOS'
(Other definitions for egos that I've seen before include "Self-images" , "People's personal pride - the parts of them that are conscious" , "feelings of self-importance" , "Lots of self-esteem" .)