Car, one appearing in King George pub sign (9)
I believe the answer is:
autograph
'sign' is the definition.
(autographing is a kind of signing)
'car one appearing in king george pub' is the wordplay.
'car' becomes 'auto' ('auto' can be a synonym of 'car').
'one' becomes 'a'.
'appearing in' indicates putting letters inside.
'king george' becomes 'gr' (Georgius Rex).
'pub' becomes 'ph' (public house).
'gr'+'ph'='grph'
'a' going into 'grph' is 'graph'.
'auto'+'graph'='AUTOGRAPH'
(Other definitions for autograph that I've seen before include "Famous person's signature" , "A signature given as souvenir" , "a collector's item perhaps" , "Signer will give this" , "(Celebrity's) signature" .)