On regular basis, nonmeat’s bad for stomachs (5)
I believe the answer is:
omasa
'for stomachs' is the definition.
(omasum is a kind of stomach)
'on regular basis nonmeat's bad' is the wordplay.
'on regular basis' means one should take alternating letters (letters selected at regular intervals).
'nonmeats'+'bad'='nonmeatsbad'
The alternate letters of 'nonmeatsbad' are 'OMASA'.
(Other definitions for omasa that I've seen before include "Organs" , "Stomachs" .)