Gallery in museum entered by monarch (7)
I believe the answer is:
veranda
'gallery' is the definition.
('veranda' can be a synonym of 'gallery')
'museum entered by monarch' is the wordplay.
'museum' becomes 'vanda'.
'entered by' means one lot of letters goes inside another (I've seen this in other clues).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'vanda' going around 'er' is 'VERANDA'.
'in' acts as a link.
(Other definitions for veranda that I've seen before include "Porch along the outside of a building" , "Roofed gallery beside a house" , "Open roofed platform" , "Roofed open terrace alongside a house" , "Covered porch on building" .)