Smallest article in case put outside (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'article in case put outside' is the wordplay.
'article' becomes 'a' (indefinite article).
'in case' becomes 'lest' (I've seen this before).
'put outside' indicates putting letters inside.
'a' going inside 'lest' is 'LEAST'.
(Other definitions for least that I've seen before include "To the lowest degree" , "Superlative form of 'little'" , "Lowest amount" , "Superlatively little" , "Bottom" .)