Letters from a peer are rather less common (5)
I believe the answer is:
rarer
'less common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'less common')
'letters from a peer are rather' is the wordplay.
'letters from' says the answer is hidden in the clue (a selection of letters from the clue).
'RARER' can be found hidden inside 'peer are rather'.
(Other definitions for rarer that I've seen before include "Less often seen" , "More scarce or uncommon" , "Stranger" , "Less frequent" , "Comparatively sparse" .)