Less familiar reindeer are rejecting harnesses (5)
I believe the answer is:
rarer
'less familiar' is the definition.
('rare' can be the opposite of 'familiar' so perhaps 'rarer' is reasonable for 'less familiar')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for rarer that I've seen before include "Less frequent" , "steak may be wanted thus?" , "Less often seen" , "Comparatively sparse" , "Thinner" .)