Regulator of company books a posh car (10)
I believe the answer is:
controller
'regulator' is the definition.
(I've seen this before)
'company books a posh car' is the wordplay.
'company' becomes 'co' (abbreviation).
'books' becomes 'nt' (New Testament).
'a posh car' becomes 'roller'.
'co'+'nt'+'roller'='CONTROLLER'
'of' is the link.
(Other definitions for controller that I've seen before include "One holding the reins" , "Regulator" .)