Make up for old head contains iodine (7)
I believe the answer is:
expiate
'make up for' is the definition.
(I've seen this in another clue)
'old head contains iodine' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'head' becomes 'pate' (humorous term for the head).
'contains' indicates putting letters inside.
'iodine' becomes 'I' (chemical symbol).
'pate' enclosing 'i' is 'piate'.
'ex'+'piate'='EXPIATE'
(Other definitions for expiate that I've seen before include "Atone for (a sin)" , "Atone for (guilt or wrongdoing)" , "Make reparation" , "Make up for mistake" , "right?" .)