Serving of butter (3)
I believe the answer is:
pat
'serving of butter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pat that I've seen before include "Light hand stroke" , "Touch lightly - bit of butter?" , "well-rehearsed" , "Some butter, maybe" , "Glib; touch affectionately" .)