No big deal, as Norman put it after mix-up (11)
I believe the answer is:
unimportant
'no big deal' is the definition.
(if something is no big deal it is unimportant)
'norman put it after mix-up' is the wordplay.
'after mix-up' is an anagram indicator.
'norman'+'put'+'it'='normanputit'
'normanputit' is an anagram of 'UNIMPORTANT'.
'as' acts as a link.
(Other definitions for unimportant that I've seen before include "Of little value or significance" , "not mattering" , "Light" .)