Blundering when dropping grand cake (6)
I believe the answer is:
muffin
'cake' is the definition.
(I've seen this before)
'blundering when dropping grand' is the wordplay.
'blundering' becomes 'muffing' ('muff' can be a synonym of 'blunder').
'when dropping' indicates named letters should be taken away.
'grand' becomes 'g' (G is short for grand as in a thousand).
'muffing' with 'g' taken away is 'MUFFIN'.
(Other definitions for muffin that I've seen before include "Flat round cake, eaten toasted and buttered (6)" , "Cake - old TV mule" , "Have IMF fun with the cake" , "Small cake" , "Round bread-like cake" .)