Not so common in wear are red socks (5)
I believe the answer is:
rarer
'not so common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'not so common')
'in wear are red socks' is the wordplay.
'in' indicates a hidden word.
'RARER' is hidden within 'wear are red socks'.
(Other definitions for rarer that I've seen before include "less easy to find" , "More scarce or uncommon" , "Less often seen" , "More unusual" , "Less frequent" .)