Observe article boil (6)
I believe the answer is:
seethe
'boil' is the definition.
(I know that boil can be written as seethe)
'observe article' is the wordplay.
'observe' becomes 'see' (observe can mean to see something).
'article' becomes 'the' (the definite article in English).
'see'+'the'='SEETHE'
(Other definitions for seethe that I've seen before include "Boil; be crowded with moving people" , "Boil; be intensely angry" , "Smoulder" , "Simmer with anger" , "Ferment, sizzle" .)