The only soldier without a grave (6)
I believe the answer is:
solemn
'grave' is the definition.
(I know that grave can be written as solemn)
'the only soldier without a' is the wordplay.
'the only' becomes 'sole' (I've seen this before).
'soldier' becomes 'man' (man can mean a soldier).
'without' indicates named letters should be taken away.
'man' with 'a' removed is 'mn'.
'sole'+'mn'='SOLEMN'
(Other definitions for solemn that I've seen before include "Serious, dignified or ceremonial" , "Marked by pomp, cermony or formality" , "Formal, ceremonial" , "Grave and serious about melons" , "Sober and serious" .)