Comparatively uncommon age for retirement in Rolls-Royce (5)
I believe the answer is:
rarer
'comparatively uncommon' is the definition.
('rare' can be similar in meaning to 'uncommon')
'age for retirement in rolls-royce' is the wordplay.
'age' becomes 'era' (both can mean a long time).
'for retirement' is a reversal indicator (retire can mean to retreat or go backwards).
'in' means one lot of letters goes inside another.
'rolls royce' becomes 'rr' (short for Rolls Royce).
'era' written backwards gives 'are'.
'are' inserted within 'rr' is 'RARER'.
(Other definitions for rarer that I've seen before include "(Of meat) less cooked" , "even less common" , "Thinner" , "Less frequent" , "Less often seen" .)