Right unit to measure land by river is more uncommon (5)
I believe the answer is:
rarer
'more uncommon' is the definition.
('rare' can be similar in meaning to 'uncommon')
'right unit to measure land by river' is the wordplay.
'right' becomes 'R' (common abbreviation).
'unit' becomes 'A' (A is an example).
'to measure land by' becomes 're' (I am not sure about this - if you are sure you should believe this answer much more).
'river' becomes 'r' (abbreviation for river).
'r'+'a'+'re'+'r'='RARER'
'is' is the link.
Can you help me to learn more?
(Other definitions for rarer that I've seen before include "Thinner" , "Comparatively sparse" , "Less often seen" , "less easy to find" , "More scarce or uncommon" .)