Service person putting iodine in join (7)
I believe the answer is:
soldier
'service person' is the definition.
'putting iodine in join' is the wordplay.
'putting iodine' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'join' becomes 'solder' (soldering is a kind of joining).
'i' placed within 'solder' is 'SOLDIER'.
Can you help me to learn more?
(Other definitions for soldier that I've seen before include "part of breakfast perhaps" , "General or private" , "serviceman" , "Enlisted man or woman" , "Person serving in army" .)