Assistant replacing one in flight — one living abroad (5)
I believe the answer is:
expat
'living abroad' is the definition.
(I've seen this before)
'assistant replacing one in' is the wordplay.
'assistant' becomes 'pa' (personal assistant).
'replacing one' becomes 'ext' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'pa' going inside 'ext' is 'EXPAT'.
'flight one' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for expat that I've seen before include "Brit abroad, say" , "Person living permanent in a foreign country" , "One who lives abroad" , "one settled abroad" , "who now uses overseas address?" .)