Open-and-shut, according to the circumstances? (2,3,4,3,2)
I believe the answer is:
as the case may be
'according to the circumstances?' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for as the case may be that I've seen before include "according to circumstances?" , "Open-and-shut, possibly" , "Depending on the particular circumstances (2,3,4,3,2)" , "Accordingly" , "according to how things turn out?" .)