Lead for canine, a very threatening thing jumping up — watch out for that! (4,5)
I believe the answer is:
cave canem
'watch out for that' is the definition.
(Latin for 'beware of the dog')
'lead for canine a very threatening thing jumping up' is the wordplay.
I cannot really understand how this works, but
'canine' could be 'c' (dental abbreviation) and 'c' is found in the answer.
'a' could be 'an' and 'an' is found in the answer.
'very' could be 'v' (abbreviation) and 'v' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Another definition for cave canem that I've seen is " Ancient warning".)