Elder served up frozen dessert and mostly grim cheese (3,9)
I believe the answer is:
red leicester
'mostly grim cheese' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'elder served up frozen dessert' is the wordplay.
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'frozen' becomes 'ice' (I've seen this in other clues).
'dessert' becomes 'ster' (I am not sure about this - if you are sure you should believe this answer much more).
'elder' reversed gives 'redle'.
'redle'+'ice'+'ster'='RED LEICESTER'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for red leicester that I've seen before include "food" , "'Cheese!'" .)