Big cheese sandwich endlessly seen in menu terribly old (6,3)
I believe the answer is:
numero uno
'big cheese' is the definition.
(both can mean an important person)
'sandwich endlessly seen in menu terribly old' is the wordplay.
'sandwich' becomes 'round' (as in a round of sandwiches).
'endlessly' means to remove the last letter.
'seen in' indicates putting letters inside.
'terribly' indicates anagramming the letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'round' with its last letter taken off is 'roun'.
'menu' is an anagram of 'nume'.
'nume'+'o'='numeo'
'roun' inserted into 'numeo' is 'NUMERO UNO'.
(Other definitions for numero uno that I've seen before include "Ace Italian" , "Big cheese from Italy" , "Berlusconi once" , "boss?" .)