Act on oral treatment for tooth cavity? (4,5)
I believe the answer is:
root canal
'tooth cavity?' is the definition.
(type of cavity in a tooth)
'act on oral treatment' is the wordplay.
'treatment' indicates anagramming the letters.
'act'+'on'+'oral'='actonoral'
'actonoral' with letters rearranged gives 'ROOT CANAL'.
'for' acts as a link.
(Other definitions for root canal that I've seen before include "oral passage" , "bit of tooth" , "Dental passage" , "area sensitive to extraction?" .)