Fit up over blunder I see - that's just great (8)
I believe the answer is:
terrific
'great' is the definition.
(I've seen this before)
'fit up over blunder i see' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'over' is an insertion indicator.
'blunder' becomes 'err' (to err is to make a mistake or blunder).
'see' becomes 'C' (the word for the letter, according to Chambers).
'fit' backwards is 'tif'.
'tif' enclosing 'err' is 'terrif'.
'terrif'+'i'+'c'='TERRIFIC'
'that's just' is the link.
(Other definitions for terrific that I've seen before include "Of great size/intensity" , "Marvellous, tremendous" , "Splendid" , "Extraordinarily great or good" , "Excellent; of great size" .)