Article unknown to those people (4)
I believe the answer is:
they
'those people' is the definition.
(I've seen this before)
'article unknown' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'unknown' becomes 'y' (algebra).
'the'+'y'='THEY'
'to' acts as a link.
(Other definitions for they that I've seen before include "People generally" , "Those people (vb. subject)" , "Pronoun for people" , "Pronoun for more than one" , "Do -- Know It's Christmas (Band Aid)" .)