Board attached to spring is so long (8)
I believe the answer is:
farewell
'is so long' is the definition.
(I've seen this before)
'board attached to spring' is the wordplay.
'board' becomes 'fare' (both can mean food provided).
'attached to' means one lot of letters go next to another.
'spring' becomes 'well' (I've seen this before).
'fare'+'well'='FAREWELL'
(Other definitions for farewell that I've seen before include "All the best" , "Wish on leave-taking" , "Departure" , "Goodbye, cheerio" , "vale" .)