Mate, I’m upset before sandwiches (5)
I believe the answer is:
amigo
'mate' is the definition.
(I've seen this in another clue)
'i'm upset before sandwiches' is the wordplay.
'upset' indicates anagramming the letters.
'before' becomes 'ago' (I've seen this before).
'sandwiches' is an insertion indicator (to sandwich is to block something in).
'im' anagrammed gives 'mi'.
'mi' inserted within 'ago' is 'AMIGO'.
(Other definitions for amigo that I've seen before include "Friend in Spanish-speaking areas" , "Hispanic comrade" , "friend in Uruguay" , "in a foreign land, my friend?" , "Friend, comrade" .)