Track vehicle circling central unit (7)
I believe the answer is:
monitor
'track' is the definition.
(to track or keep an eye on)
'vehicle circling central unit' is the wordplay.
'vehicle' becomes 'motor' (motor vehicle is a kind of vehicle).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'central' says to take the centre.
The centre of 'unit' is 'ni'.
'motor' placed around 'ni' is 'MONITOR'.
(Other definitions for monitor that I've seen before include "Sort of lizard" , "Prefect; a loudspeaker" , "Checking device - VDU - lizard" , "Large lizard; TV screen" , "Police" .)