Queen travelling by rail in country (7)
I believe the answer is:
terrain
'country' is the definition.
(terrain means some land or country)
'queen travelling by rail' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'travelling by' is an insertion indicator (some letters travel within others).
'rail' becomes 'train' (I can't explain this - if you can you should give a lot more credence to this answer).
'er' inserted within 'train' is 'TERRAIN'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for terrain that I've seen before include "Physical feature of a stretch of ground" , "Tract of land with particular character" , "A stretch of land for trainer" , "Landscape" , "Land (viewed tactically)" .)