Gallery about to be erected in museum (7)
I believe the answer is:
veranda
'gallery' is the definition.
(synonyms)
'about to be erected in museum' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'to be erected' is a reversal indicator (in a down clue, letters go up).
'in' indicates putting letters inside.
'museum' becomes 'V and A' (Victoria and Albert Museum).
're' reversed gives 'er'.
'er' placed into 'vanda' is 'VERANDA'.
(Other definitions for veranda that I've seen before include "Roofed platform level with the ground floor" , "Ground-floor extension" , "Roofed open terrace alongside a house" , "Sheltered terrace around a house" , "Open roofed platform" .)