Most significant account about capital of Italy in history (7)
I believe the answer is:
archaic
'in history' is the definition.
'most significant account about capital of italy' is the wordplay.
'most significant' becomes 'arch' (as in archbishop, arch enemy etc).
'account' becomes 'ac'.
'about' means one lot of letters goes inside another.
'capital of' says to take the initial letters.
The initial letter of 'italy' is 'i'.
'ac' enclosing 'i' is 'aic'.
'arch'+'aic'='ARCHAIC'
(Other definitions for archaic that I've seen before include "No longer in general use" , "Belonging to former times like some words" , "From the past" , "Primitive" , "Ancient, no longer current" .)