Sort of prime side of skin (9)
I believe the answer is:
epidermis
'skin' is the definition.
(I've seen this before)
'sort of prime side' is the wordplay.
'sort of' is an anagram indicator.
'prime'+'side'='primeside'
'primeside' is an anagram of 'EPIDERMIS'.
'of' acts as a link.
(Other definitions for epidermis that I've seen before include "Outermost layer" , "Protective covering" , "Surface of the skin" , "Outer layer of skin" .)