Track going round a river -- it's something unusual (6)
I believe the answer is:
rarity
'something unusual' is the definition.
(I've seen this in another clue)
'track going round a river it's' is the wordplay.
'track' becomes 'ry' (abbreviation for railway).
'going round' is an insertion indicator (some letters go around others).
'river' becomes 'r' (abbreviation).
'a'+'r'+'it'='arit'
'ry' enclosing 'arit' is 'RARITY'.
(Other definitions for rarity that I've seen before include "Infrequent occurrence" , "Uncommon thing" , "Find" , "Thinness" , "Unusual item" .)