Soft chap, solemn at heart (6)
I believe the answer is:
gentle
'soft' is the definition.
(I know that soft can be written as gentle)
'chap solemn at heart' is the wordplay.
'chap' becomes 'gent' (both can mean a man).
'at heart' means to look at the middle letters.
The middle letters of 'solemn' are 'le'.
'gent'+'le'='GENTLE'
(Other definitions for gentle that I've seen before include "Moderate, quiet; a maggot" , "Mild, kind, tender" , "Soft and mild, quiet and soothing" , "that may help angler catch fish" , "Mild, not rough" .)