Silly, but it has a twin, possibly (6)
I believe the answer is:
nitwit
'silly' is the definition.
(thesaurus)
'it has a twin possibly' is the wordplay.
'has' means one lot of letters go next to another.
'possibly' indicates anagramming the letters.
'it'+'twin'='ittwin'
'ittwin' anagrammed gives 'NITWIT'.
'but' is the link.
(Other definitions for nitwit that I've seen before include "Informally a stupid person, like funny flea?" , "Foolish person like funny louse?" , "Silly person (colloq.)" , "Twin it with a right eejit" , "Simpleton" .)