Danger! Initially vicious, aggressive chihuahua's turned up? (4,5)
I believe the answer is:
cave canem
'danger initially vicious aggressive chihuahua's turned up?' is the definition.
I can't tell whether this definition defines the answer.
'danger initially vicious aggressive chihuahua's turned up?' is the wordplay.
'danger' becomes 'menace' (menace is a kind of danger).
'initially' indicates taking the first letters.
'turned up?' says the letters should be written backwards.
The initial letters of 'vicious aggressive chihuahuas' is 'vac'.
'menace'+'vac'='menacevac'
'menacevac' reversed gives 'CAVE CANEM'.
Can you help me to learn more?
(Another definition for cave canem that I've seen is " Ancient warning".)