Activity of browser in operating system, so mad? (6,8)
I believe the answer is:
window shopping
'activity of browser' is the definition.
'operating system so mad?' is the wordplay.
'operating system' becomes 'windows'.
'so mad?' becomes 'hopping' (ref to the expression 'hopping mad').
'windows'+'hopping'='WINDOW-SHOPPING'
'in' is the link.
(Other definitions for window shopping that I've seen before include "as browser is doing" , "Browsing" .)