Posh females splitting fish dish (7)
I believe the answer is:
souffle
'dish' is the definition.
(souffle is a kind of dish)
'posh females splitting fish' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'females' becomes 'ff' (an abbreviation for female written twice).
'splitting' is an insertion indicator (some letters split a word into two).
'fish' becomes 'sole' (sole is a kind of fish).
'u'+'ff'='uff'
'uff' put into 'sole' is 'SOUFFLE'.
(Other definitions for souffle that I've seen before include "light dish is presented" , "Dish made of egg yolks and stiffly beaten egg whites" , "Frothy dish made with egg whites" , "light baking" , "Light fluffy dish with a base of egg yolks and stiffly beaten egg whites" .)