Smallest article enclosed just in case (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'article enclosed just in case' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'enclosed' is an insertion indicator.
'just in case' becomes 'lest' (I've seen this in another clue).
'a' placed within 'lest' is 'LEAST'.
(Other definitions for least that I've seen before include "Smallest in amount or degree" , "Smallest (in significance)" , "Lowest amount" , "Superlatively little" , "Most unimportant" .)