Mark, good person, regularly patient (5)
I believe the answer is:
stain
'mark' is the definition.
(synonyms)
'good person regularly patient' is the wordplay.
'good person' becomes 'st' (saint).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'patient' are 'ain'.
'st'+'ain'='STAIN'
(Other definitions for stain that I've seen before include "Mark with colour, dirt" , "Blot" , "Satin (anag.)" , "Blemish; colour (wood)" , "Discoloured mark" .)