Terrace of museum visited by the Queen (7)
I believe the answer is:
veranda
'terrace' is the definition.
(I've seen this before)
'museum visited by the queen' is the wordplay.
'museum' becomes 'vanda'.
'visited by' is an insertion indicator.
'the queen' becomes 'er' (Elizabeth Regina).
'vanda' enclosing 'er' is 'VERANDA'.
'of' acts as a link.
(Other definitions for veranda that I've seen before include "Porch - portico" , "Roofed terrace along the side of a house" , "Roofed open gallery" , "Sheltered terrace around a house" , "Stoop" .)