Seldom encountered river measurement unit (4)
I believe the answer is:
rare
'seldom encountered' is the definition.
(I've seen this in another clue)
'measurement unit' is the wordplay.
'measurement' becomes 'r' (r is a kind of measurement).
'unit' becomes 'are' (are is a kind of unit).
'r'+'are'='RARE'
'river' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "Seen once in a blue moon" , "thin on the ground" , "(Of meat) red" , "like gold dust" , "Scarce or uncommon" .)