Foreign aide unhappy, at first, to be so long in Paris (5)
I believe the answer is:
adieu
'be so long in paris' is the definition.
('goodbye' or 'so long' in French)
'foreign aide unhappy at first' is the wordplay.
'foreign' indicates anagramming the letters (foreign order of the letters).
'at first' indicates taking the first letters.
The initial letter of 'unhappy' is 'u'.
'aide' is an anagram of 'adie'.
'adie'+'u'='ADIEU'
'to' acts as a link.
(Other definitions for adieu that I've seen before include "cheers" , "Farewell, ami!" , "farewell message" , "Goodbye in France" , "12 Down" .)