Beginning to pen line for article (6)
I believe the answer is:
clause
'for article' is the definition.
(synonyms)
'beginning to pen line' is the wordplay.
'beginning' becomes 'cause' (cause is a kind of beginning).
'to pen' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cause' placed around 'l' is 'CLAUSE'.
(Other definitions for clause that I've seen before include "Joker, say" , "Verb-carrying component of sentence" , "Joker, eg" , "Subunit of a sentence" , "Subdivision of a sentence" .)