Fill mug up (4)
I believe the answer is:
cram
I believe this clue is a double definition.
'fill' is the first definition.
('cram' can be a synonym of 'fill')
'mug up' is the second definition.
('cram' can be a synonym of 'mug up')
(Other definitions for cram that I've seen before include "jam" , "Quickly learn - stuff" , "Study hard for exam" , "Crowd or pack to capacity" , "Stuff; study hard" .)