Bit by bit, go inside controlling mechanism in parts of country (7)
I believe the answer is:
regions
'parts of country' is the definition.
(a region is a large part of a country)
'bit by bit go inside controlling mechanism in' is the wordplay.
'bit by' becomes 'o' (resembles 0, a binary digit or bit. I am not sure about the 'by' bit.).
'bit' suggests taking the first letters (I've seen 'bits of' mean this).
'inside' is an insertion indicator.
'controlling mechanism' becomes 'reins' (means of controlling a horse).
'in' means one lot of letters goes inside another.
The initial letter of 'go' is 'g'.
'g' placed inside 'reins' is 'regins'.
'o' inserted inside 'regins' is 'REGIONS'.
(Other definitions for regions that I've seen before include "patches" , "Ignores (anag.)" , "Extended areas" , "Domains" , "'Areas, locations (7)'" .)