Cadfael regularly follows Saint Peter (4)
I believe the answer is:
safe
'peter' is the definition.
(slang term)
'cadfael regularly follows saint' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'follows' is a charade indicator (letters next to each other).
'saint' becomes 's'.
The alternate letters of 'cadfael' are 'afe'.
'afe' put after 's' is 'SAFE'.
(Other definitions for safe that I've seen before include "Uninjured, secure" , "Metal box for storing valuables in" , "Cash-box" , "Unharmed" , "Secure in metal box?" .)