Lacking iodine, badly infected exterior (9)
I believe the answer is:
deficient
'lacking' is the definition.
(I know that lacking can be written as deficient)
'iodine badly infected exterior' is the wordplay.
'iodine' becomes 'I' (chemical symbol).
'badly' indicates anagramming the letters (letters in the wrong order).
'exterior' indicates putting letters inside (some letters go on the outside of others).
'infected' is an anagram of 'deficent'.
'i' put into 'deficent' is 'DEFICIENT'.
(Other definitions for deficient that I've seen before include "Lacking" , "Inadequate" , "less than complete" .)