Earnest single male losing heart (6)
I believe the answer is:
solemn
'earnest' is the definition.
(thesaurus)
'single male losing heart' is the wordplay.
'single' becomes 'sole' ('sole' can be a synonym of 'single').
'male' becomes 'man' (man is a kind of male).
'losing heart' suggests removing the centre.
'man' with its middle taken out is 'mn'.
'sole'+'mn'='SOLEMN'
(Other definitions for solemn that I've seen before include "Sober and serious" , "Marked by pomp, cermony or formality" , "Marked by pomp or ceremony" , "Formal, ceremonius" , "gloomy" .)