Reputed to be unhappy about frailty at heart? (4)
I believe the answer is:
said
'reputed to be' is the definition.
(I've seen this in another clue)
'unhappy about frailty at heart?' is the wordplay.
'unhappy' becomes 'sad' (similar in meaning).
'about' means one lot of letters goes inside another.
'at heart?' means to look at the middle letters (I've seen 'at heart' mean this).
The central letter of 'frailty' is 'i'.
'sad' going around 'i' is 'SAID'.
(Other definitions for said that I've seen before include "Aforementioned" , "Mentioned" , "Spoke" , "Remarked" , "... gave expression" .)