Regularly leave euro? Always (4)
I believe the answer is:
ever
'always' is the definition.
('ever' can be a synonym of 'always')
'regularly leave euro?' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'leave'+'euro'='leaveeuro'
The alternate letters of 'leaveeuro' are 'EVER'.
(Other definitions for ever that I've seen before include "constantly" , "Perpetually" , "In perpetuity" , "Eternally" , "To the end of time" .)