Unfriendly idiot stands up after end of opera (5)
I believe the answer is:
aloof
'unfriendly' is the definition.
(I've seen this in another clue)
'idiot stands up after end of opera' is the wordplay.
'idiot' becomes 'fool' (I've seen this in another clue).
'stands up' says the letters should be written backwards (in a down clue, letters go up).
'after' is a charade indicator (letters next to each other).
'end of' says to take the final letters.
The last letter of 'opera' is 'a'.
'fool' back-to-front is 'loof'.
'loof' put after 'a' is 'ALOOF'.
(Other definitions for aloof that I've seen before include "rather cold" , "Unapproachable, distant" , "Standing apart" , "Distant in manner" , "Disinterested" .)