Pack a quarter in the choicest part (5)
I believe the answer is:
cream
'choicest part' is the definition.
(I've seen this in another clue)
'pack a quarter in' is the wordplay.
'pack' becomes 'cram' (synonyms).
'a quarter' becomes 'e'.
'in' indicates putting letters inside.
'cram' placed around 'e' is 'CREAM'.
'the' acts as a link.
(Other definitions for cream that I've seen before include "Thick milk" , "best people" , "The best part -of the crop" , "Butterfat part of milk" , "Best part of the milk" .)