Declare again, "Fear mixed business!" (8)
I believe the answer is:
reaffirm
'declare again' is the definition.
('affirm' is an answer for 'declare' so perhaps 'reaffirm' is OK for 'declare again')
'fear mixed business' is the wordplay.
'mixed' indicates anagramming the letters.
'business' becomes 'firm' (firm is a kind of business).
'fear' is an anagram of 'reaf'.
'reaf'+'firm'='REAFFIRM'
(Other definitions for reaffirm that I've seen before include "Assert again" , "Once again, insist" , "State again strongly" , "Declare to be true, again" .)