Likeness shows Posh in mess (6)
I believe the answer is:
statue
'likeness' is the definition.
(I have seen 'Sculptured likeness ' mean 'statue' so perhaps 'likeness' could also mean 'statue')
'posh in mess' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'in' is an insertion indicator.
'mess' becomes 'state' (I am not sure about this - if you are sure you should believe this answer much more).
'u' going into 'state' is 'STATUE'.
'shows' is the link.
Can you help me to learn more?
(Other definitions for statue that I've seen before include "Still work" , "Solid representation of the human form" , "Eg, the Venus de Milo" , "Figure, effigy" , "Carved likeness" .)