Are in red, possibly, when costlier (6)
I believe the answer is:
dearer
'costlier' is the definition.
('dear' can be a synonym of 'costly')
'are in red possibly' is the wordplay.
'in' indicates putting letters inside.
'possibly' indicates an anagram.
'red' is an anagram of 'der'.
'are' placed into 'der' is 'DEARER'.
'when' acts as a link.
(Other definitions for dearer that I've seen before include "More expensive or loved" , "Less cheap" , "More expensive or more beloved" , "Nearer to the heart" .)