Ground vehicle carrying the Queen (7)
I believe the answer is:
terrain
'ground' is the definition.
(thesaurus)
'vehicle carrying the queen' is the wordplay.
'vehicle' becomes 'train'.
'carrying' is an insertion indicator.
'the queen' becomes 'er' (Elizabeth Regina).
'train' going around 'er' is 'TERRAIN'.
(Other definitions for terrain that I've seen before include "Topography" , "Piece of ground" , "Trainer (anag) - land" , "Tract of land with particular character" , "Stretch of land of a particular sort" .)