Serious regrets over show-off (6)
I believe the answer is:
poseur
'show-off' is the definition.
(I know that poseur is a type of show-off)
'serious regrets over' is the wordplay.
'serious' becomes 'po' (I can't explain this - if you can you should believe this answer much more).
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'over' is a reversal indicator.
'rues' backwards is 'seur'.
'po'+'seur'='POSEUR'
Can you help me to learn more?
(Other definitions for poseur that I've seen before include "One shows off to impress" , "Pseud, posturer" , "Pretentious person" , "Attitudiniser (Fr.)" , "Uprose (anag.)" .)