Smallest amount went off, stale (5)
I believe the answer is:
least
'smallest amount' is the definition.
(I've seen this before)
'went off stale' is the wordplay.
'went off' indicates anagramming the letters.
'stale' is an anagram of 'LEAST'.
(Other definitions for least that I've seen before include "Smallest part" , "Smallest (in significance)" , "Smallest, slightest" , "Being lowest in importance - gone stale" , "Superlatively little" .)