Only chap losing heart is unsmiling (6)
I believe the answer is:
solemn
'unsmiling' is the definition.
(I've seen this before)
'only chap losing heart' is the wordplay.
'only' becomes 'sole' (synonyms).
'chap' becomes 'Man' (I've seen this before).
'losing heart' means to remove the middle letters.
'man' with its centre removed is 'mn'.
'sole'+'mn'='SOLEMN'
'is' acts as a link.
(Other definitions for solemn that I've seen before include "Serious, dignified or ceremonial" , "Portentous" , "Formal, ceremonius" , "Marked by pomp or ceremony" , "Stately, pompous or glum" .)