Manage commercial unit in church (10)
I believe the answer is:
administer
'manage' is the definition.
(administering is a kind of managing)
'commercial unit in church' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'unit' becomes 'i' (synonyms).
'in' indicates putting letters inside.
'church' becomes 'minster' (minster is a kind of church).
'i' inserted into 'minster' is 'minister'.
'ad'+'minister'='ADMINISTER'
(Other definitions for administer that I've seen before include "Deal out (punishment)" , "control" , "conduct" , "Handle" , "Manage" .)