Choice morsel of Brie? Precisely (5)
I believe the answer is:
plumb
'precisely' is the definition.
(plumb can mean exactly or precisely)
'choice morsel of brie?' is the wordplay.
'choice' becomes 'plum' (both can mean excellent).
'morsel of' suggests taking the first letters (only a small amount).
The first letter of 'brie' is 'b'.
'plum'+'b'='PLUMB'
(Other definitions for plumb that I've seen before include "Lead weight for testing depth or the perpendicular" , "Absolutely straight - sound" , "Lead weight; exactly" , "Precisely" , "Dead" .)