Reputed to be unhappy about one (4)
I believe the answer is:
said
'reputed to be' is the definition.
(I've seen this in another clue)
'unhappy about one' is the wordplay.
'unhappy' becomes 'sad' (synonyms).
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'sad' going around 'i' is 'SAID'.
(Other definitions for said that I've seen before include "Aforementioned" , "Spoken" , "Announced; Egypt. Port" , "Uttered at Suez port" , "Stated" .)