Most sore, a thorax injected with iodine (7)
I believe the answer is:
achiest
'most sore' is the definition.
'a thorax injected with iodine' is the wordplay.
'thorax' becomes 'chest' (synonyms).
'injected with' means one lot of letters goes inside another.
'iodine' becomes 'I' (I is the chemical symbol for iodine).
'chest' going around 'i' is 'chiest'.
'a'+'chiest'='ACHIEST'
(Other definitions for achiest that I've seen before include "most painful" , "suffering most pain" .)