Cast-iron commercial progress (6)
I believe the answer is:
inroad
'progress' is the definition.
(thesaurus)
'cast-iron commercial' is the wordplay.
'cast' indicates an anagram (letters are dispersed or cast around).
'commercial' becomes 'ad' (abbreviation for advertisement).
'iron' anagrammed gives 'inro'.
'inro'+'ad'='INROAD'
(Other definitions for inroad that I've seen before include "Foray" , "Encroachment; hostile attack" , "Encroachment, intrusion" , "Offensive" , "Ordain (anag.)" .)