Skin gashes need iodine putting in (5)
I believe the answer is:
cutis
'skin' is the definition.
(I know that skin can be written as cutis)
'gashes need iodine putting in' is the wordplay.
'gashes' becomes 'cuts' ('cut' can be a synonym of 'gash').
'need iodine' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'putting in' means one lot of letters goes inside another.
'cuts' enclosing 'i' is 'CUTIS'.
Can you help me to learn more?
(Other definitions for cutis that I've seen before include "skin, technically" , "Skin, dermis" , "Technical name for skin" , "a bit skinny" , "Surface area" .)