Hide and seek: extremely popular (4)
I believe the answer is:
Here is my best explanation:
'hide' is the definition.
('skin' can be a synonym of 'hide')
'seek extremely popular' is the subsidiary indication.
'extremely' means to remove the middle letters.
'popular' becomes 'in'.
'seek' with its middle taken out is 'sk'.
'and' is the link.
(Other definitions for skin that I've seen before include "Outer layer of a fruit or vegetable", "hide", "Personal cover?", "Flense", "Swindle", "Epidermis - peel", "Scrape, as the knee", "Outer layer of, eg, apple", "Remove the outer layer" and "Thin outer layer".)