Trainer reorganised ground (7)
I believe the answer is:
terrain
'ground' is the definition.
(thesaurus)
'trainer reorganised' is the wordplay.
'reorganised' indicates an anagram.
'trainer' with letters rearranged gives 'TERRAIN'.
(Other definitions for terrain that I've seen before include "Tract of land with particular character" , "Piece of ground" , "Type of countryside" , "Ground (conditions)" , "Land (viewed tactically)" .)