Ogre mad to eat posh cosmetic (5)
I believe the answer is:
rouge
'cosmetic' is the definition.
(rouge is a kind of cosmetic)
'ogre mad to eat posh' is the wordplay.
'mad' indicates anagramming the letters.
'to eat' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'ogre' anagrammed gives 'roge'.
'roge' going around 'u' is 'ROUGE'.
(Other definitions for rouge that I've seen before include "Book used in church" , "Red face powder" , "See 1" , "Ferric oxide powder" , "Polish used by jewellers" .)