Sea setback, unfortunately, has been cast a crippling blow (6,4)
I believe the answer is:
basket case
'been cast a crippling blow' is the definition.
I can't judge whether this defines the answer.
'sea setback unfortunately' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'sea'+'setback'='seasetback'
'seasetback' anagrammed gives 'BASKET CASE'.
'has' is the link.
Can you help me to learn more?
(Other definitions for basket case that I've seen before include "economically weak country" , "A nervous wreck" , "Person unable to function properly" , "Very poor country" , "hopeless type" .)