The smallest amount, incredibly stale (5)
I believe the answer is:
least
'the smallest amount' is the definition.
(I've seen this before)
'incredibly stale' is the wordplay.
'incredibly' indicates anagramming the letters.
'stale' with letters rearranged gives 'LEAST'.
(Other definitions for least that I've seen before include "Most unimportant" , "Smallest (in significance)" , "Fewest" , "Minimal amount" , "Superlatively little" .)