Just in case the article therein is the smallest (5)
I believe the answer is:
least
'the smallest' is the definition.
(I've seen this in another clue)
'just in case the article therein' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'article' could be 'a' (the indefinite article in English) and 'a' is found in the answer.
The remaining letters 'les' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for least that I've seen before include "Of minimal significance" , "Minimal amount" , "Superlative form of 'little'" , "Smallest (in significance)" , "Lowest amount" .)