Slips the waiter four euros at the end (4)
I believe the answer is:
errs
'slips' is the definition.
('err' can be a synonym of 'slip')
'the waiter four euros at the end' is the wordplay.
'at the end' suggests the final letters.
The final letters of 'the waiter four euros' are 'ERRS'.
(Other definitions for errs that I've seen before include "Makes mistakes" , "Goes wrong" , "Boobs" , "Slips up" , "Goes astray" .)