Queen opening museum: horse gallery (8)
I believe the answer is:
verandah
'gallery' is the definition.
(I know that gallery can be written as verandah)
'queen opening museum horse' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'opening' is an insertion indicator (some letters open a gap in others).
'museum' becomes 'V and A' (Victoria and Albert Museum).
'horse' becomes 'h' (synonyms).
'er' placed into 'vanda' is 'veranda'.
'veranda'+'h'='VERANDAH'
(Other definitions for verandah that I've seen before include "Roofed area on the side of a house" , "Porch" , "Portico" , "16 21 [INDIAN IMPORT]" , "gallery" .)