On reflection, I might add, a cold-blooded killer (3)
I believe the answer is:
asp
'cold-blooded killer' is the definition.
(I've seen this before)
'on reflection i might add a' is the wordplay.
'on reflection' says the letters should be written backwards.
'i might add' becomes 'ps'.
'ps'+'a'='psa'
'psa' backwards is 'ASP'.
(Other definitions for asp that I've seen before include "A glider's what's called for" , "One secreting poison" , "Venomous snake" , "Cleopatra's poisoner" , "Egyptian cobra" .)