Everyman rues regularly being simple (4)
I believe the answer is:
mere
'being simple' is the definition.
('mere' can be a synonym of 'simple')
'everyman rues regularly' is the wordplay.
'everyman' becomes 'me' (where the crossword setter is 'Everyman').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'rues' are 're'.
'me'+'re'='MERE'
(Other definitions for mere that I've seen before include "Nothing more than is specified - or a poetic lake" , "Pool, small lake" , "Pure; lake" , "President Hollande's mother?" , "Lake (poetic); Maori war club" .)