Ground used by unconventional trainer (7)
I believe the answer is:
terrain
'ground' is the definition.
(terrain is land or ground)
'unconventional trainer' is the wordplay.
'unconventional' indicates an anagram.
'trainer' anagrammed gives 'TERRAIN'.
'used by' acts as a link.
(Other definitions for terrain that I've seen before include "A stretch of land for trainer" , "Trainer (anag) - land" , "Land (viewed tactically)" , "Ground - rare tin (anag)" , "Land of a particular type" .)