Mum twice trapping black snake (5)
I believe the answer is:
mamba
'snake' is the definition.
(I know that Mamba is a type of elapid snake)
'mum twice trapping black' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'twice' indicates the doubling of some letters.
'trapping' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'ma' duplicated is 'mama'.
'mama' placed around 'b' is 'MAMBA'.
(Other definitions for mamba that I've seen before include "Venomous Afr. snake" , "It may be deadly" , "One's venomous" , "Poisonous creature" , "venomous crawler" .)