Mother embraces British crawler (5)
I believe the answer is:
mamba
'crawler' is the definition.
(I have seen 'venomous crawler' mean 'mamba' so perhaps 'crawler' could also mean 'mamba')
'mother embraces british' is the wordplay.
'mother' becomes 'mama' (mama informally means mother).
'embraces' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'mama' enclosing 'b' is 'MAMBA'.
(Other definitions for mamba that I've seen before include "one venomous" , "Venomous Afr. snake" , "Large African snake" , "Venomous African tree snake" , "Venomous African snake (often black)" .)