Manufacturer regularly signed American apprentice (13)
I believe the answer is:
industrialist
'manufacturer' is the definition.
(thesaurus)
'regularly signed american apprentice' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'american' becomes 'us'.
'apprentice' becomes 'trialist' (a young employee e.g. in a football team).
The alternate letters of 'signed' are 'ind'.
'ind'+'us'+'trialist'='INDUSTRIALIST'
(Other definitions for industrialist that I've seen before include "Krupp, eg" , "Ford, perhaps" , "Eg, factory owner" , "Manufacturing magnate" , "enterprising type" .)