King George the Sixth appearing in poster with child (6)
I believe the answer is:
gravid
'with child' is the definition.
(gravid means pregnant)
'king george the sixth appearing in poster' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'the sixth' becomes 'VI' (Roman numerals).
'appearing in' indicates putting letters inside.
'poster' becomes 'ad' (short for advert).
'vi' going inside 'ad' is 'avid'.
'gr'+'avid'='GRAVID'
(Other definitions for gravid that I've seen before include "Expecting" , "Pregnant or heavy" , "(In medicine) pregnant" , "In an advanced stage of pregnancy" .)