Article about first large plant (7)
I believe the answer is:
thistle
'plant' is the definition.
(type of plant)
'article about first large' is the wordplay.
'article' becomes 'the' (the definite article in English).
'about' is an insertion indicator.
'first' becomes 'ist' (ist resembles 1st).
'large' becomes 'L' (eg in clothes sizes).
'ist'+'l'='istl'
'the' placed around 'istl' is 'THISTLE'.
(Other definitions for thistle that I've seen before include "The Scottish weed?" , "that blows down when it's windy" , "Scotland's flower" , "Prickly plant with purple flowers" , "Prickly plant (Carduus)" .)