Rare being casually dressed in court? Carry on (2,5,6)
I believe the answer is:
in short supply
'rare' is the definition.
(thesaurus)
'casually dressed in court? carry on' is the wordplay.
I cannot quite understand how this works, but
'in' is found in the answer.
'on' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'being' is the link.
(Other definitions for in short supply that I've seen before include "Not enough to satisfy demand" , "Rarely seen" , "Not often seen" , "Scarce" .)