Food all right for so long (8)
I believe the answer is:
farewell
'so long' is the definition.
(I've seen this before)
'food all right' is the wordplay.
'food' becomes 'fare' (fare is a kind of food**).
'all right' becomes 'well' ('well' can be a synonym of 'all right').
'fare'+'well'='FAREWELL'
'for' is the link.
(Other definitions for farewell that I've seen before include "vale" , "Goodbye, adieu, so long" , "Archaic goodbye - good luck" , "Parting word" , "Say goodbye" .)