Terrain lost by instructor (7)
I believe the answer is:
trainer
'instructor' is the definition.
(thesaurus)
'terrain lost' is the wordplay.
'lost' indicates an anagram.
'terrain' anagrammed gives 'TRAINER'.
'by' is the link.
(Other definitions for trainer that I've seen before include "Casual shoe" , "Footwear singularly" , "Sports footwear" , "Sports instructor" , "Shoe; instructor" .)