Admit office-holder is endlessly foul-mouthed (5,2)
I believe the answer is:
Here is my best explanation:
'admit office-holder' is the definition.
'endlessly foul-mouthed' is the subsidiary indication.
'endlessly' means to remove the last letter.
'foul-mouthed' becomes 'swearing'.
'swearing' with its final letter taken away is 'swearin'.
'is' is the link.
(Other definitions for swear in that I've seen before include "Inaugurate by oath" and "Administer oath to one taking up office".)