Regular advance carries forward head of state (7)
I believe the answer is:
monarch
'head of state' is the definition.
(monarch is a type of head of state)
'regular advance carries forward' is the wordplay.
'regular advance' becomes 'march' (as in the march of history).
'carries' is an insertion indicator (inserted letters carried inside).
'forward' becomes 'on'.
'march' enclosing 'on' is 'MONARCH'.
(Other definitions for monarch that I've seen before include "Not a subject" , "Crowned head" , "Eg, king, queen, emperor" , "Butterfly - ruler" , "one over realm?" .)