Respect to cast is only shown under exceptional circumstances (3-6)
I believe the answer is:
top-secret
'only shown under exceptional circumstances' is the definition.
I can't judge whether this definition defines the answer.
'respect to cast' is the wordplay.
'cast' is an anagram indicator (letters are dispersed or cast around).
'respect'+'to'='respectto'
'respectto' anagrammed gives 'TOP-SECRET'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for top-secret that I've seen before include "Erect post (anag)" , "Highest classified (information)" , "Most confidential , not to be leaked on any account" , "Very hush-hush" , "Highly classified" .)