Tear drop regularly includes one (3)
I believe the answer is:
rip
'tear' is the definition.
('rip' can be a synonym of 'tear')
'drop regularly includes one' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'includes' indicates putting letters inside.
'one' becomes 'i' (Roman numeral**).
The alternate letters of 'drop' are 'rp'.
'rp' enclosing 'i' is 'RIP'.
(Other definitions for rip that I've seen before include "Letters usually carved on gravestone" , "parting message" , "Tearaway" , "Rest in peace" , "Tear or split open" .)