Unaccompanied, the Parisian up on top of Eiger (5)
I believe the answer is:
alone
'unaccompanied' is the definition.
('alone' can be a synonym of 'unaccompanied')
'the parisian up on top of eiger' is the wordplay.
'the parisian' becomes 'la' (French for the).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'top of' says to take the initial letters.
The first letter of 'eiger' is 'e'.
'la' backwards is 'al'.
'al'+'on'+'e'='ALONE'
(Other definitions for alone that I've seen before include "Lacking company, solitary" , "unique" , "Without others being involved" , "Exclusively; solitary" , "without another" .)