Journey down is fair with hint of sun breaking in (7)
I believe the answer is:
Here is my best explanation:
'journey down' is the definition.
(I've seen this before)
'fair with hint of sun breaking in' is the wordplay.
I cannot quite understand how this works, but
'sun' could be 's' and 's' is found within the answer.
'fair' could be 'decent' and 'decent' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
(Other definitions for descent that I've seen before include "Lineage", "Downward move", "Bloodline", "Journey down", "Ancestry - downward move", "Downward journey", "A climb down", "Downward slope", "Pedigree" and "Fall".)