Proceed, as in “proceed against” (5)
I believe the answer is:
segue
'proceed' is the definition.
(I know that segue is a more specific form of the action proceed)
'as in proceed against' is the wordplay.
'as' becomes 'eg' ('as' and 'e.g.' can both mean for example).
'in' indicates putting letters inside.
'proceed against' becomes 'sue' (synonyms).
'eg' put inside 'sue' is 'SEGUE'.
(Other definitions for segue that I've seen before include "Change smoothly from one thing to another" , "Follow smoothly on" , "Play on" , "Go on without interruption" , "In music, an immediate transition from one part to another" .)