Principal ingredient of regular charcoal (4)
I believe the answer is:
arch
'principal' is the definition.
(thesaurus)
'ingredient of regular charcoal' is the wordplay.
'ingredient of' indicates a hidden word.
'ARCH' can be found hidden inside 'regular charcoal'.
(Other definitions for arch that I've seen before include "Coyly playful" , "Prefix meaning chief - maybe an angel" , "Marble, for example" , "part of fingerprint" , "one under a foot?" .)