Plain wholesome ready-made sandwiches (4)
I believe the answer is:
mere
'plain' is the definition.
('mere' can be similar in meaning to 'plain')
'wholesome ready-made sandwiches' is the wordplay.
'sandwiches' indicates the answer is hidden within the clue (the solution is sandwiched in by the clue).
'MERE' can be found hidden inside 'wholesome ready made'.
(Other definitions for mere that I've seen before include "sea" , "Without additions or modifications" , "Lake (and nothing else?)" , "Lake (poetic); Maori war club" , "Small area of standing water" .)