Upper-class snake caught finally as killer down under (1-4)
I believe the answer is:
u-boat
'killer down under' is the definition.
(German war submarine)
'upper-class snake caught finally' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'snake' becomes 'boa' (boa is a kind of snake).
'finally' indicates one should take the final letters.
The final letter of 'caught' is 't'.
'u'+'boa'+'t'='U-BOAT'
'as' acts as a link.
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "Take pool exercise" , "German WW 2 sub" , "German wartime submarine" , "Submersible warship armed with torpedoes" .)