Hot drink's knocked over plug (4)
I believe the answer is:
push
'plug' is the definition.
(I know that plug is a more specific form of the action push)
'hot drink's knocked over' is the wordplay.
'hot' becomes 'h' (common abbreviation eg on taps).
'drink' becomes 'sup' (to sup is to drink).
'knocked over' says the letters should be written backwards.
'h'+'sup'='hsup'
'hsup' backwards is 'PUSH'.
(Other definitions for push that I've seen before include "Drive forward" , "Encourage" , "Promote (commodity)" , "Energy" , "Press forward" .)