From now on a shade retiring and I'm not surprised (2,6)
I believe the answer is:
no wonder
'i'm not surprised' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'from now on a shade retiring' is the wordplay.
'from' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'a shade' becomes 'red' (red is a colour or shade).
'retiring' shows that the letters should be reversed in order.
'red' written backwards gives 'der'.
'on' after 'now' is 'nowon'.
'nowon'+'der'='NO WONDER'
'and' is the link.
Can you help me to learn more?
(Other definitions for no wonder that I've seen before include "it isn't surprising" , "hardly unexpected" , "hardly unforeseen" .)