Concerned with component of blood containing iodine — that’s worrying (7)
I believe the answer is:
serious
'worrying' is the definition.
(serious can mean worrying or dangerous)
'concerned with component of blood containing iodine' is the wordplay.
'concerned with component of blood' becomes 'serous' (relating to serum).
'containing' means one lot of letters goes inside another.
'iodine' becomes 'I' (I is the chemical symbol for iodine).
'serous' placed around 'i' is 'SERIOUS'.
'that's' is the link.
(Other definitions for serious that I've seen before include "Not light or frivolous" , "Earnest, sincere" , "Important -- grave" , "Solemn, thoughtful" , "Important - solemn" .)