Tongue I wash off with iodine after first of lesions (7)
I believe the answer is:
swahili
'tongue' is the definition.
(Swahili is a language)
'i wash off with iodine after first of lesions' is the wordplay.
'off with' is an anagram indicator.
'iodine' becomes 'I' (chemical symbol).
'after' means one lot of letters go next to another.
'first of lesions' becomes 'l' (1st letter of 'lesions').
'i'+'wash'='iwash'
'i'+'l'='il'
'iwash' anagrammed with 'il' gives 'SWAHILI'.
(Other definitions for swahili that I've seen before include "heard in parts of Africa" , "African people" , "Major Bantu language widely used in E Africa" , "Widely-spoken Bantu language" , "An official language spoken in 15" .)