In red, perhaps, one looks terrible! (4)
I believe the answer is:
dire
'terrible' is the definition.
(synonyms)
'in red perhaps one' is the wordplay.
'in' means one lot of letters goes inside another.
'perhaps' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'red' with letters rearranged gives 'dre'.
'dre' enclosing 'i' is 'DIRE'.
'looks' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for dire that I've seen before include "Fearful" , "Desperate; dreadful" , "Terrible display" , "Ride off in dreadful way" , "to speak (13) [FRENCH]" .)