Manage ably for so long (8)
I believe the answer is:
farewell
'for so long' is the definition.
(both mean goodbye)
'manage ably' is the wordplay.
'manage' becomes 'fare' (how you fare is how you manage with something).
'ably' becomes 'we'll' (to do something well is to do it ably or skilfully).
'fare'+'well'='FAREWELL'
(Other definitions for farewell that I've seen before include "Wish on leave-taking" , "Archaic goodbye or leave-taking" , "Cape at the southernmost point of Greenland" , "All the best" , "Goodbye, cheerio" .)