Clearly superior cave about to collapse (1,3,5)
I believe the answer is:
a cut above
'clearly superior' is the definition.
'cave about to collapse' is the wordplay.
'to collapse' indicates anagramming the letters.
'cave'+'about'='caveabout'
'caveabout' with letters rearranged gives 'A CUT ABOVE'.
(Other definitions for a cut above that I've seen before include "Noticeably superior to" , "Something superior" .)