Formidable woman’s uncle shakily penning article (9)
I believe the answer is:
herculean
'formidable' is the definition.
(thesaurus)
'woman's uncle shakily penning article' is the wordplay.
'woman's' becomes 'her' (belonging to a woman).
'shakily' indicates an anagram.
'penning' indicates putting letters inside.
'article' becomes 'a' (English indefinite article).
'uncle' anagrammed gives 'culen'.
'culen' enclosing 'a' is 'culean'.
'her'+'culean'='HERCULEAN'
(Other definitions for herculean that I've seen before include "Enormously demanding (task)" , "Showing great strength" , "So enormous a task" , "Monumental (task)" , "for Poirot?" .)