The smallest amount of arsenic stored by permit (5)
I believe the answer is:
least
'the smallest amount' is the definition.
(I've seen this before)
'arsenic stored by permit' is the wordplay.
'arsenic' becomes 'as' (As is the chemical symbol for arsenic).
'stored by' indicates putting letters inside.
'permit' becomes 'let' (synonyms).
'as' inserted inside 'let' is 'LEAST'.
'of' is the link.
(Other definitions for least that I've seen before include "No more petty" , "Opposite of most" , "Most slight" , "Of minimal significance" , "Most insignificant" .)