It's understood I whip up dessert at the end (5)
I believe the answer is:
tacit
'it's understood' is the definition.
(I've seen this before)
'i whip up dessert at the end' is the wordplay.
'whip' becomes 'cat' (cat o' nine tails, cat for short is a type of whip).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'at the end' suggests the final letters.
The last letter of 'dessert' is 't'.
'i'+'cat'='icat'
'icat' reversed gives 'taci'.
'taci'+'t'='TACIT'
(Other definitions for tacit that I've seen before include "Implicitly understood" , "Wordless" , "Implied, unstated" , "Understood, not spoken or played" , "Unvoiced" .)