About to go up inside museum gallery? (7)
I believe the answer is:
veranda
'gallery?' is the definition.
('veranda' can be a synonym of 'gallery')
'about to go up inside museum' is the wordplay.
'about' becomes 're' (regarding**).
'to go up' shows that the letters should be reversed in order (I've seen 'goes up' mean this).
'inside' means one lot of letters goes inside another.
'museum' becomes 'vanda'.
're' back-to-front is 'er'.
'er' put into 'vanda' is 'VERANDA'.
(Other definitions for veranda that I've seen before include "Roofed open terrace alongside a house" , "Ground-floor extension" , "Porch along the outside of a building" , "Roofed open gallery" , "Porch - portico" .)