Threaten to bring elf to a conclusion (6)
I believe the answer is:
impend
'threaten' is the definition.
(I've seen this in another clue)
'elf to a conclusion' is the wordplay.
'elf' becomes 'imp' (synonyms).
'to a conclusion' becomes 'end' (something's conclusion is its end).
'imp'+'end'='IMPEND'
'to bring' acts as a link.
(Other definitions for impend that I've seen before include "appearing to be imminent" , "Threaten; approach" , "seen to be around the corner" , "menace" , "Threaten - be about to happen" .)