Princess is getting on plane at last (6)
I believe the answer is:
isolde
'princess' is the definition.
(legendary princess)
'is getting on plane at last' is the wordplay.
'getting on' becomes 'old' ('getting on a bit' means 'old').
'at last' suggests the final letters.
The last letter of 'plane' is 'e'.
'is'+'old'+'e'='ISOLDE'
(Other definitions for isolde that I've seen before include "Soiled like Tristram's girl friend" , "heroine (sic)" , "Soiled like Tristan's lover" , "Wagner heroine" , "Legendary lover" .)