King George has a double but not all at once (7)
I believe the answer is:
gradual
'not all at once' is the definition.
(I've seen this in another clue)
'king george has a double' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'has' means one lot of letters go next to another.
'double' becomes 'dual' (synonyms).
'gr'+'a'+'dual'='GRADUAL'
'but' is the link.
(Other definitions for gradual that I've seen before include "Continuous" , "By easy stages, not sudden or steep" , "(Of slope) not steep" , "Step by step" , "mass response" .)