Food that is rotten (5)
I believe the answer is:
scoff
'food' is the definition.
(I've seen this before)
'that is rotten' is the wordplay.
'that is' becomes 'sc' (abbrv. for Latin scilicet; 'namely').
'rotten' becomes 'off' (I've seen this before).
'sc'+'off'='SCOFF'
(Other definitions for scoff that I've seen before include "turtle soup for example?" , "Jeer; eat greedily" , "Furiously swallow" , "Mock" , "taunt" .)