Some deer are rather more uncommon (5)
I believe the answer is:
rarer
'more uncommon' is the definition.
('rare' can be similar in meaning to 'uncommon')
'some deer are rather' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'RARER' can be found hidden inside 'deer are rather'.
(Other definitions for rarer that I've seen before include "More scarce or uncommon" , "steak may be wanted thus?" , "Less frequent" , "Stranger" , "More unusual" .)