About to include one French mystic symbol (4)
I believe the answer is:
rune
'mystic symbol' is the definition.
(I've seen this before)
'about to include one french' is the wordplay.
'about' becomes 're' (regarding**).
'to include' indicates putting letters inside.
'one french' becomes 'un'.
're' enclosing 'un' is 'RUNE'.
(Other definitions for rune that I've seen before include "Letter of an ancient Germanic alphabet" , "Ancient symbol" , "Old alphabet letter" , "Old Scandinavian letter" , "Early symbol used in carvings" .)