Take up or do in a suitable case (5)
I believe the answer is:
adopt
'take up' is the definition.
(I know that take up can be written as adopt)
'do in a suitable case' is the wordplay.
'in' is an insertion indicator.
'a suitable case' becomes 'apt' (I am not sure about this - if you are sure you should believe this answer much more).
'do' placed inside 'apt' is 'ADOPT'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for adopt that I've seen before include "Take up, follow" , "Take responsibility for" , "Formally approve" , "make one's own" , "Take into one's family" .)