This is proving difficult. The anagram at 10 down occupied a great deal of time before the penny dropped. I am struggling with 6d "Exhibition's opening Windows in small chamber lacking length" . I have ?UER??ON but cannot parse any answer from the possibles. Neither have I the three word phrase but Shackleton might be a lead.
there was a misprint in the clues. 6d should be in Italics and is "Exhibition's opening".
7d is "Windows ...... length". That should help.
Look away now if you don't want a hint (below) to the 3 word phrase.