Old lady captivates a doctor with snake (5)
I believe the answer is:
Here is my best explanation:
'snake' is the definition.
(mamba is a kind of snake)
'old lady captivates a doctor' is the subsidiary indication.
'old lady' becomes 'ma' (**).
'captivates' is an insertion indicator.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'ma' going around 'amb' is 'mamba'.
'with' is the link.
(Other definitions for mamba that I've seen before include "Poisonous creature", "Large African snake", "Deadly creature", "one venomous", "serpent", "It may be deadly", "Snake that might kill", "Deadly African snake", "Venomous African snake (often black)" and "venomous crawler".)