Possible Clues:

• Place to display art
• Anagram of 54-Across's ending that can follow the first word of 20-, 33-, 40- and 54-Across
• Covered walk
• Place where pictures are exhibited
• Largely (anag.)
• Extended passage of river in boat ...
• Theatre area
• Art display where one may see a lot of rogues?
• Building for works of art
• Eg, the Tate