Admit police mostly represented talk about official case (10,3)
I believe the answer is:
diplomatic bag
'official case' is the definition.
(container for official documents)
'admit police mostly represented talk about' is the wordplay.
'mostly' means to remove the last letter.
'represented' indicates an anagram.
'talk' becomes 'gab' (I've seen this in another clue).
'about' says the letters should be written backwards.
'gab' backwards is 'bag'.
'admit'+'police'='admitpolice'
'admitpolice' with its final letter removed is 'admitpolic'.
'admitpolic' is an anagram of 'diplomatic'.
'diplomatic'+'bag'='DIPLOMATIC BAG'
(Other definitions for diplomatic bag that I've seen before include "Embassy mail sack" , "One taking post abroad" , "Embassy's mail container" , "Embassy mail pouch" .)