Case of rickets interrupting show is a setback (8)
I believe the answer is:
reversal
'a setback' is the definition.
('reversal' can be a synonym of 'setback')
'case of rickets interrupting show' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'interrupting' indicates putting letters inside.
'show' becomes 'reveal' (revealing is a kind of showing).
'rickets' with its middle taken out is 'rs'.
'rs' inserted into 'reveal' is 'REVERSAL'.
'is' acts as a link.
(Other definitions for reversal that I've seen before include "Turnaround - defeat" , "U-turn" , "that doesn't affect mum or dad" , "Turnabout" , "Volte-face" .)