As stated above they requested a VMC descent, they were never CLEARED for anything.
Not sure what you mean, or in what form you would want a "Clearance".
Chippendales words
On the Captain’s instructions the co-pilot asked “McMurdo Centre” for a clearance to let down VMC to 2,000 feet and proceed visually to McMurdo. Immediately after this approval was given a further descent was commenced. Report 79-139
and
ATC did give a clearance to descend to 2,000 ft VMC and proceed visually to McMurdo. Royal Commission