Cold-blooded killers slip away, quietly heading south (4)
I believe the answer is:
asps
'cold-blooded killers' is the definition.
(I've seen this in another clue)
'slip away quietly heading south' is the wordplay.
'slip away' becomes 'as' (I can't justify this - if you can you should give a lot more credence to this answer).
'quietly' becomes 'p' (musical symbol).
'heading south' becomes 's' (abbreviation).
'as'+'p'+'s'='ASPS'
Can you help me to learn more?
(Other definitions for asps that I've seen before include "because of their venom?" , "Small vipers" , "Poisonous creatures" , "Cleopatra's cobras" , "Pass (anag)" .)