Commercial assistance provided by governing body (14)
I believe the answer is:
administration
'governing body' is the definition.
(thesaurus)
'commercial assistance' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'assistance' becomes 'ministration' (ministration is a kind of assistance).
'ad'+'ministration'='ADMINISTRATION'
'provided by' is the link.
(Other definitions for administration that I've seen before include "government in power" , "Running a business" .)