Part of the terrain the Scots own (3)
I believe the answer is:
ain
'own' is the definition.
(synonyms)
'part of the terrain the scots' is the wordplay.
'part of' indicates a hidden word.
'AIN' is hidden within 'terrain scots'.
(Other definitions for ain that I've seen before include "Own (Scot)" , "Glaswegian personal" , "The Scot's own" , "Your own in Scottish dialect" , "26 [ ]" .)