It's poisonous so mother gets a doctor in (5)
I believe the answer is:
mamba
'it's poisonous' is the definition.
(venomous snake)
'mother gets a doctor in' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'gets' says to put letters next to each other.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' indicates putting letters inside.
'ma'+'a'='maa'
'maa' enclosing 'mb' is 'MAMBA'.
'so' is the link.
(Other definitions for mamba that I've seen before include "Large African snake" , "Venomous African tree snake" , "Venomous Afr. snake" , "Venomous snake; rhymes with dance" , "Venomous African snake (often black)" .)