Gallery constructed in a royal manner (7)
I believe the answer is:
regally
'in a royal manner' is the definition.
(I know this)
'gallery constructed' is the wordplay.
'constructed' indicates anagramming the letters.
'gallery' is an anagram of 'REGALLY'.
(Other definitions for regally that I've seen before include "In monarchical manner" , "Like a monarch" , "How head of state appears" , "Largely in royal manner" , "as befits monarch" .)