Devotee regularly in cult (3)
I believe the answer is:
nut
'devotee' is the definition.
(thesaurus)
'regularly in cult' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in'+'cult'='incult'
The alternate letters of 'incult' are 'NUT'.
(Other definitions for nut that I've seen before include "Somebody obsessed" , "Bolt's partner" , "Hazel; beech" , "Fanatical enthusiast" , "Brazil, for instance" .)