Silly fool gets virus shot (9)
I believe the answer is:
frivolous
'silly' is the definition.
(similar in meaning)
'fool gets virus shot' is the wordplay.
'gets' means one lot of letters go next to another.
'shot' indicates anagramming the letters.
'fool'+'virus'='foolvirus'
'foolvirus' with letters rearranged gives 'FRIVOLOUS'.
(Other definitions for frivolous that I've seen before include "Unserious" , "Trifling, silly" , "Carefree, superficial" , "Trivial - flippant" , "Carefree and not serious" .)