A drive out every so often can be so long (5)
I believe the answer is:
'be so long' is the definition.
(both can mean goodbye)
'a drive out every so often' is the wordplay.
'every so often' indicates alternate letters.
The alternate letters of 'driveout' are 'dieu'.
'can' acts as a link.
(Other definitions for adieu that I've seen before include "Goodbye to the French" , "Comment on leaving" , "left, making this?" , "Goodbye, madame" , "Leave" .)