After injection of iodine, a thorax is most sore (7)
I believe the answer is:
achiest
'most sore' is the definition.
(I've seen this in another clue)
'after injection of iodine a thorax' is the wordplay.
'after injection of' indicates putting letters inside.
'iodine' becomes 'I' (chemical symbol).
'thorax' becomes 'chest' (synonyms).
'a'+'chest'='achest'
'i' inserted inside 'achest' is 'ACHIEST'.
'is' acts as a link.
(Other definitions for achiest that I've seen before include "suffering most pain" , "most painful" .)