Lumberjack’s warning doctor to get in line (6)
I believe the answer is:
timber
'lumberjack's warning' is the definition.
(a lumberjack shouts 'timber!' before a tree falls)
'doctor to get in line' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'to get in' is an insertion indicator.
'line' becomes 'tier' (tier is a kind of line).
'mb' placed within 'tier' is 'TIMBER'.
(Other definitions for timber that I've seen before include "Watch out for falling tree!" , "wooden" , "Mind the falling tree!" , "The tree's falling!" , "Warning cry that a tree is about to fall" .)