Splendidly grand in a genuine way (7)
I believe the answer is:
regally
'splendidly' is the definition.
Both the answer and definition are adverbs. Perhaps you can see an association between them that I can't see?
'grand in a genuine way' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'in' means one lot of letters goes inside another.
'a genuine way' becomes 'really' (I've seen this before. I am not sure about the 'genuine' bit.).
'g' going inside 'really' is 'REGALLY'.
Can you help me to learn more?
(Other definitions for regally that I've seen before include "as befits monarch" , "in great style" , "How head of state appears" , "like a head of state" , "In royal style" .)