Cold-blooded American being busy leader (10)
I believe the answer is:
copperhead
'cold-blooded american' is the definition.
(type of snake found in North America)
'busy leader' is the wordplay.
'busy' becomes 'copper' (I've seen this before).
'leader' becomes 'head' (head is a kind of leader).
'copper'+'head'='COPPERHEAD'
'being' is the link.
(Other definitions for copperhead that I've seen before include "Reptile" , "venomous sort" , "Policeman brains snake" , "Venomous, reddish-brown American snake" , "Highly venomous US or Australian snake" .)