Illustration features, mainly, type of skin disease (6)
I believe the answer is:
Here is my best explanation:
'illustration features mainly type of skin disease' is the definition.
I can't judge whether this definition defines the answer.
'illustration features mainly type' is the wordplay.
'illustration' becomes 'pic' (short for picture).
'features' means one lot of letters goes inside another.
'mainly' means to remove the last letter.
'type' becomes 'sort' (I've seen this before).
'sort' with its final letter taken off is 'sor'.
'pic' going around 'sor' is 'PSORIC'.
Can you help me to learn more?