Only man to lose a grave (6)
I believe the answer is:
solemn
'grave' is the definition.
('solemn' can be a synonym of 'grave')
'only man to lose a' is the wordplay.
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'to lose' suggests deleting specific letters.
'man' with 'a' removed is 'mn'.
'sole'+'mn'='SOLEMN'
(Other definitions for solemn that I've seen before include "gloomy" , "Formal, dignified" , "Portentous" , "Unsmiling" , "Formal, ceremonial" .)