Understand an article to be in red (4)
I believe the answer is:
read
'understand' is the definition.
(read can mean to understand or analyse)
'an article to be in red' is the wordplay.
'an article' becomes 'a' (the indefinite article in English).
'to be in' is an insertion indicator.
'a' put into 'red' is 'READ'.
(Other definitions for read that I've seen before include "See - peruse - understand" , "Lecture" , "Comprehend written words" , "Absorbed (written text)" , "Study (for a degree?)" .)