Huge area included in visits regularly (4)
I believe the answer is:
vast
'huge' is the definition.
(I know that huge can be written as vast)
'area included in visits regularly' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'included in' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'visits' are 'vst'.
'a' going into 'vst' is 'VAST'.
(Other definitions for vast that I've seen before include "Unusually great" , "Boundless" , "'Huge, immense (4)'" , "Astronomical" , "Colossal" .)