Container companion filled with iodine (4)
I believe the answer is:
pail
'container' is the definition.
(I know that pail is a type of vessel)
'companion filled with iodine' is the wordplay.
'companion' becomes 'pal' (I've seen this in another clue).
'filled with' means one lot of letters goes inside another.
'iodine' becomes 'I' (I is the chemical symbol for iodine).
'pal' going around 'i' is 'PAIL'.
(Other definitions for pail that I've seen before include "Cylindrical vessel" , "A bucket - the kind Jack and Jill used" , "Water carrier" , "Milkmaid's bucket?" , "water-carrier" .)