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' means one lot of letters goes inside another.
'possibly' indicates anagramming the letters.
'red' is an anagram of 'der'.
'are' inserted inside 'der' is 'DEARER'.
'when' is the 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" .)