Company perhaps regularly ruins city centre (4)
I believe the answer is:
unit
'company perhaps' is the definition.
(I know that caller is a type of visitant)
'regularly ruins city centre' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'city centre' becomes 'it' (centre of the word 'city').
The alternating letters of 'ruins' are 'un'.
'un'+'it'='UNIT'
(Other definitions for unit that I've seen before include "Detachment of soldiers" , "Any division of quantity accepted as a standard of measurement" , "Building block - small military force" , "A considerable number of soldiers" , "Single manufactured item; military grouping" .)