Best part of the milk (5)
I believe the answer is:
cream
This clue is a triple definition!
'best' is the first definition.
(as in 'the cream of the crop')
'part' is the second definition.
(I have seen 'Best part ' mean 'cream' so perhaps 'part' could also mean 'cream')
'of' is a link.
'the milk' is the third definition.
(I have seen 'Thick milk ' mean 'cream' so perhaps 'milk' could also mean 'cream')
(Other definitions for cream that I've seen before include "treat for cat?" , "'There are more ways of killing a cat than choking it with ....." , "Top or best part" , "Pick - fatty part of milk" , "Best - ointment" .)