Inside, truly grand in a majestic way (7)
I believe the answer is:
regally
'in a majestic way' is the definition.
(I know this)
'inside truly grand' is the wordplay.
'inside' means one lot of letters goes inside another.
'truly' becomes 'really' (synonyms).
'grand' becomes 'g' (G is short for grand as in a thousand).
'really' placed around 'g' is 'REGALLY'.
(Other definitions for regally that I've seen before include "Largely in royal manner" , "In royal style" , "as befits monarch" , "In royal fashion" , "How head of state appears" .)