Mum catching black serpent (5)
I believe the answer is:
mamba
'serpent' is the definition.
(I know that Mamba is a type of elapid)
'mum catching black' is the wordplay.
'mum' becomes 'mama' (synonyms).
'catching' 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 "Deadly African snake" , "Venomous snake; rhymes with dance" , "Venomous Afr. snake" , "one venomous" , "Venomous African tree snake" .)