The smallest amount could be stale (5)
I believe the answer is:
least
'the smallest amount' is the definition.
(I've seen this before)
'could be stale' is the wordplay.
'could be' indicates an anagram.
'stale' is an anagram of 'LEAST'.
(Other definitions for least that I've seen before include "Bottom" , "Lowest or smallest" , "No more petty" , "Superlative form of 'little'" , "Minimum" .)