Do as others do to understand case (6,4)
I believe the answer is:
follow suit
'do as others do' is the definition.
(I've seen this before)
'understand case' is the wordplay.
'understand' becomes 'follow'.
'case' becomes 'suit' ('suit' can be a synonym of 'case').
'follow'+'suit'='FOLLOW SUIT'
'to' is the link.
(Other definitions for follow suit that I've seen before include "go from club to club" , "Play another club, perhaps" , "Do the same as someone else" , "Do as others do" , "in due course" .)