Show up a bygone attitude (6)
I believe the answer is:
expose
'show up' is the definition.
(I've seen this before)
'a bygone attitude' is the wordplay.
'a bygone' becomes 'ex' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'ex'+'pose'='EXPOSE'
Can you help me to learn more?
(Other definitions for expose that I've seen before include "Lay out to view" , "article on scandal?" , "hazard" , "Reveal, uncover" , "brings out crime" .)