Hide and seek: extremely popular (4)
I believe the answer is:
skin
'hide' is the definition.
('skin' can be a synonym of 'hide')
'seek extremely popular' is the wordplay.
'extremely' means to remove the middle letters.
'popular' becomes 'in'.
'seek' with its centre removed is 'sk'.
'sk'+'in'='SKIN'
'and' is the link.
(Other definitions for skin that I've seen before include "Epidermis - peel" , "Thin outer layer" , "Fell" , "Outer layer of a fruit or vegetable" , "Swindle" .)