Beneath French castle, munched rib and steak (13)
I believe the answer is:
chateaubriand
'steak' is the definition.
(Chateaubriand is an example)
'beneath french castle munched rib and' is the wordplay.
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'french castle' becomes 'chateau' ('castle' in French).
'munched' is an anagram indicator (letters munched or crushed into a new order).
'rib' with letters rearranged gives 'bri'.
'chateau' put next to 'bri' is 'chateaubri'.
'chateaubri'+'and'='CHATEAUBRIAND'
(Other definitions for chateaubriand that I've seen before include "French writer" , "cut" , "Fr. writer; fillet steak" , "French author" , "Thick beef steak fillet" .)