Medic introduced to swimmer in wasteland (6)
I believe the answer is:
tundra
'wasteland' is the definition.
(I've seen this before)
'medic introduced to swimmer' is the wordplay.
'medic' becomes 'dr' (doctor).
'introduced to' indicates putting letters inside.
'swimmer' becomes 'tuna' (tuna is a kind of aquatic vertebrate).
'dr' placed inside 'tuna' is 'TUNDRA'.
'in' acts as a link.
(Other definitions for tundra that I've seen before include "wasteland" , "Area of permanently frozen subsoil" , "Vast flat treeless zone" , "Vast level Arctic region with frozen subsoil" , "Treeless northern region" .)