Mother holds black snake (5)
I believe the answer is:
mamba
'snake' is the definition.
(I know that Mamba is a type of elapid)
'mother holds black' is the wordplay.
'mother' becomes 'mama' (mama informally means mother).
'holds' means one lot of letters goes inside another.
'black' becomes 'b' (abbreviation as in pencil hardness).
'mama' enclosing 'b' is 'MAMBA'.
(Other definitions for mamba that I've seen before include "one venomous" , "It may be deadly" , "Venomous snake; rhymes with dance" , "Death-dealing African" , "Venomous African tree snake" .)