I'm off food and water (8)
I believe the answer is:
farewell
'i'm off' is the definition.
'food and water' is the wordplay.
'food' becomes 'fare' (fare is a kind of food).
'and' means one lot of letters go next to another.
'water' becomes 'well' (well water is a kind of water).
'fare'+'well'='FAREWELL'
(Other definitions for farewell that I've seen before include "Cape at the southernmost point of Greenland" , "Goodbye, hope you get on" , "Archaic goodbye or leave-taking" , "Archaic goodbye - good luck" , "Valedictory word" .)