Say nothing of parent’s promise to pen article (4,3,4)
I believe the answer is:
mums the word
'say nothing' is the definition.
(phrase used to request discretion or secrecy)
'parent's promise to pen article' is the wordplay.
'parent's' becomes 'mum's' (mum means a mother).
'promise' becomes 'word' (as in 'I give you my word').
'to pen' means one lot of letters goes inside another.
'article' becomes 'the' (the definite article in English).
'mums'+'word'='mumsword'
'mumsword' enclosing 'the' is 'MUMS THE WORD'.
'of' acts as a link.
(Other definitions for mums the word that I've seen before include "Say nothing!" , "What do you call your mother" , "What's parent called" , "Keep silent (colloq.)" , "Secrecy demanded or promised" .)