A drive out every so often can be so long (5)
I believe the answer is:
adieu
'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.
'drive'+'out'='driveout'
The alternate letters of 'driveout' are 'dieu'.
'a'+'dieu'='ADIEU'
'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" .)