Fish circling river -- it's not a common thing (6)
I believe the answer is:
rarity
'not a common thing' is the definition.
(a rarity is uncommon)
'fish circling river it's' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'circling' is an insertion indicator (circling can mean going around or containing).
'river' becomes 'r' (abbreviation for river).
'r'+'it'='rit'
'ray' enclosing 'rit' is 'RARITY'.
(Other definitions for rarity that I've seen before include "Thinness" , "Unusual specimen" , "Find" , "Infrequent occurrence" , "Something not often seen" .)