European doctor on American board (5)
I believe the answer is:
embus
'board' is the definition.
(I've seen this before)
'european doctor on american' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'american' becomes 'US' (United States).
'e'+'mb'+'us'='EMBUS'
(Other definitions for embus that I've seen before include "Board" , "prepare for journey" .)