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 wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'wants' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
(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".)