"A false premise," I'd put in, "when it comes to skin" (9)
I believe the answer is:
epidermis
'skin' is the definition.
(I've seen this in another clue)
'a false premise i'd put in' is the wordplay.
'a false' is an anagram indicator.
'put in' is an insertion indicator.
'premise' anagrammed gives 'epermis'.
'epermis' placed around 'id' is 'EPIDERMIS'.
'when it comes to' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for epidermis that I've seen before include "Outermost layer" , "Protective covering" , "Outer layer of skin" , "Surface of the skin" .)