One that's in credit could be colder (5)
I believe the answer is:
icier
'colder' is the definition.
('icy' can be similar in meaning to 'cold')
'one that's in credit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'that's' becomes 'ie'.
'in' means one lot of letters goes inside another.
'credit' becomes 'cr'.
'ie' going inside 'cr' is 'cier'.
'i'+'cier'='ICIER'
'could be' is the link.
(Other definitions for icier that I've seen before include "with more reserve" , "much less friendly" , "More glacial" , "Even more frozen" , "More slippery" .)