Show up a former attitude (6)
I believe the answer is:
expose
'show up' is the definition.
(I've seen this before)
'a former attitude' is the wordplay.
'a former' becomes 'ex' (prefix meaning former or onetime).
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'ex'+'pose'='EXPOSE'
(Other definitions for expose that I've seen before include "outing" , "Show up" , "Abandon" , "Lay out to view" , "Make visible or obvious" .)