Billy's got to hide an article (4)
I believe the answer is:
goat
'billy's' is the definition.
(thesaurus)
'got to hide an article' is the wordplay.
'to hide' indicates putting letters inside.
'an article' becomes 'a' (the indefinite article in English).
'got' going around 'a' is 'GOAT'.
(Other definitions for goat that I've seen before include "nanny, for example" , "foolish person" , "heavenly sign" , "Butting nanny, perhaps?" , "Animal in zodiac" .)