Desire imp to change outer layer of skin (9)
I believe the answer is:
epidermis
'outer layer of skin' is the definition.
(I've seen this before)
'desire imp to change' is the wordplay.
'to change' indicates an anagram.
'desire'+'imp'='desireimp'
'desireimp' is an anagram of 'EPIDERMIS'.
(Other definitions for epidermis that I've seen before include "Surface of the skin" , "Outermost layer" , "Protective covering" , "Outer layer of skin" .)