Admit office-holder is endlessly foul-mouthed (5,2)
I believe the answer is:
swear in
'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 "prepare to deliver evidence" , "Get into office" , "Inaugurate by oath" , "Administer official oath" , "Administer oath to one taking up office" .)