Have an obligation to participate in rising casework (3)
I believe the answer is:
owe
'have an obligation' is the definition.
(to owe someone is to have an obligation to them)
'participate in rising casework' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'participate in' indicates the answer is hidden within the clue (I've seen 'participating in' mean this).
'ewo' can be found hidden inside 'casework'.
'ewo' written backwards gives 'OWE'.
'to' is the link.
(Other definitions for owe that I've seen before include "Have outstanding" , "Need to pay" , "Have obligation to pay" , "Have to thank" , "Be beholden" .)