Plant doctors on board train going north (6)
I believe the answer is:
mimosa
'plant' is the definition.
The answer is an example.
'doctors on board train going north' is the wordplay.
'doctors' becomes 'MOS' (an MO is a Medical Officer).
'on board' means one lot of letters goes inside another (inserted letters are taken on board).
'train' becomes 'aim' (synonyms).
'going north' says the letters should be written in reverse.
'aim' in reverse letter order is 'mia'.
'mos' placed within 'mia' is 'MIMOSA'.
(Other definitions for mimosa that I've seen before include "Flower of Maoism?" , "Tropical plant" , "Type of acacia tree" , "Yellow-flowered acacia tree" , "Tropical tree or shrub with clusters of fragrant flowers" .)