American doctor wants old desk (4)
I believe the answer is:
Here is my best explanation:
'desk' is the definition.
'american doctor wants old' is the subsidiary indication.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'wants' says to put letters next to each other.
'old' becomes 'o'.
(Other definitions for ambo that I've seen before include "desk", "Early Christian pulpit", "Raised pulpit", "Pulpit", "Early form of pulpit", "Where the preacher stood" and "Old reading-desk".)