Forced smile eating posh breakfast (6)
I believe the answer is:
muesli
'breakfast' is the definition.
(common breakfast food)
'forced smile eating posh' is the wordplay.
'forced' indicates an anagram.
'eating' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'smile' with letters rearranged gives 'mesli'.
'mesli' placed around 'u' is 'MUESLI'.
(Other definitions for muesli that I've seen before include "Mule is fed with breakfast compound" , "Breakfast mix of oats, nuts, fruit etc" , "Mixture of grain and dried fruit" , "Breakfast cereal mix" , "Breakfast mixture of oats, nuts, fruit etc" .)