Occasionally heard in museum gallery (7)
I believe the answer is:
veranda
'gallery' is the definition.
(synonyms)
'occasionally heard in museum' is the wordplay.
'occasionally' means one should take alternating letters.
'in' means one lot of letters goes inside another.
'museum' becomes 'V and A' (Victoria and Albert Museum).
The alternating letters of 'heard' are 'er'.
'er' inserted inside 'vanda' is 'VERANDA'.
(Other definitions for veranda that I've seen before include "Part of house roofed" , "Porch - portico" , "Roofed platform level with the ground floor" , "Covered porch on building" , "balcony" .)