Mother holding black snake (5)
I believe the answer is:
mamba
'snake' is the definition.
(I know that Mamba is a type of elapid)
'mother holding black' is the wordplay.
'mother' becomes 'mama' (mama informally means mother).
'holding' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'mama' enclosing 'b' is 'MAMBA'.
(Other definitions for mamba that I've seen before include "It may be deadly" , "Poisonous creature" , "Snake that might kill" , "Deadly African snake" , "Large African snake" .)