Only man to lose heart sober (6)
I believe the answer is:
solemn
'sober' is the definition.
('solemn' can be a synonym of 'sober')
'only man to lose heart' is the wordplay.
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'to lose heart' suggests removing the centre (I've seen 'losing heart' mean this).
'man' with its middle taken out is 'mn'.
'sole'+'mn'='SOLEMN'
(Other definitions for solemn that I've seen before include "Sober and serious" , "'Dignified, sombre (6)'" , "Unsmiling" , "Serious, dignified or ceremonial" , "Serious and formal" .)