The sun on one is a comfort (6)
I believe the answer is:
solace
'comfort' is the definition.
(I know that comfort can be written as solace)
'the sun on one' is the wordplay.
'the sun' becomes 'sol' (Latin for 'sun').
'on' says to put letters next to each other (in a down clue, letters appear on others).
'one' becomes 'ace' (ace has a value of one in some card games).
'sol'+'ace'='SOLACE'
'is a' is the link.
(Other definitions for solace that I've seen before include "Relief in affliction" , "Something that comforts distress" , "Consolation, comfort" , "Comfort in time of disappointment or sorrow" , "Comfort at a time of sadness" .)