Raising hat, single chap is earnest (8)
I believe the answer is:
diligent
'earnest' is the definition.
(thesaurus)
'raising hat single chap' is the wordplay.
'raising' shows that the letters should be reversed in order (in a down clue, letters go up).
'hat' becomes 'lid' (lid can informally mean a hat).
'single' becomes 'I' (one).
'chap' becomes 'gent' (both can mean a man).
'lid' in reverse letter order is 'dil'.
'dil'+'i'+'gent'='DILIGENT'
'is' is the link.
(Other definitions for diligent that I've seen before include "Showing care and effort" , "Steady and hard-working" , "Putting care and effort into work" , "Thorough" , "Steady in application and industry" .)