Austrian composer about to enter another European country after upset (4)
I believe the answer is:
berg
'austrian composer' is the definition.
(I've seen this in another clue)
'to enter another european country after upset' is the wordplay.
'to enter' is an insertion indicator.
'another' becomes 'r' (I've seen this in other clues).
'european' becomes 'E' (abbreviation e.g. EU).
'country' becomes 'GB' (Great Britain).
'after upset' shows that the letters should be reversed in order (in a down clue, letters go up).
'r'+'e'='re'
're' put within 'gb' is 'greb'.
'greb' backwards is 'BERG'.
'about' acts as a link.
(Other definitions for berg that I've seen before include "famously cut short a transatlantic journey" , "Large mass of floating ice" , "One who scored" , "Possible shipping hazard" , "Small version of ice mountain" .)