Inflexible doctor needing documentation (5)
I believe the answer is:
rigid
'inflexible' is the definition.
(I know that inflexible can be written as rigid)
'doctor needing documentation' is the wordplay.
'doctor' becomes 'rig' (I've seen this in other clues).
'needing' is a charade indicator (letters next to each other).
'documentation' becomes 'ID' (identification documents).
'rig'+'id'='RIGID'
(Other definitions for rigid that I've seen before include "Firmly fixed" , "not relaxing" , "Hard" , "Stiff and inflexible" , "completely" .)