Single man, heartlessly serious (6)
I believe the answer is:
solemn
'serious' is the definition.
(similar in meaning)
'single man heartlessly' is the wordplay.
'single' becomes 'sole' (similar in meaning).
'heartlessly' says to hollow out the word (remove centre letters).
'man' with its centre taken out is 'mn'.
'sole'+'mn'='SOLEMN'
(Other definitions for solemn that I've seen before include "Serious and humourless" , "Portentous" , "Serious, dignified or ceremonial" , "Formal, dignified" , "Formal, ceremonius" .)