King turned whitish before extremely severe deterioration in health (7)
I believe the answer is:
relapse
'deterioration in health' is the definition.
(I've seen this before)
'king turned whitish before extremely severe' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'turned' shows that the letters should be reversed in order.
'whitish' becomes 'pale' (synonyms).
'before' means one lot of letters go next to another.
'extremely' says to hollow out the word (remove centre letters).
'severe' with its middle taken out is 'se'.
'pale' back-to-front is 'elap'.
'r'+'elap'+'se'='RELAPSE'
(Other definitions for relapse that I've seen before include "Become ill again" , "Fall back into former state" , "Sink back into illness" , "A return to illness" , "unfortunate medical state" .)