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 wordplay.
'endlessly' means to remove the last letter.
'foul-mouthed' becomes 'swearing'.
'swearing' with its final letter removed is 'SWEAR IN'.
'is' acts as a link.
(Other definitions for swear in that I've seen before include "Get into office", "Inaugurate by oath", "Administer oath to one taking up office", "prepare to deliver evidence", "Formally admit", "Admit to office", "Induct into office", "bring candidate to office" and "Administer official oath".)