Cowboy made rapid progress, he being in credit (7)
I believe the answer is:
rancher
'cowboy' is the definition.
(I know that cowboy can be written as rancher)
'made rapid progress he being in credit' is the wordplay.
'made rapid progress' becomes 'ran' (to run is to move rapidly).
'being in' indicates putting letters inside.
'credit' becomes 'cr' (abbreviation).
'he' put inside 'cr' is 'cher'.
'ran'+'cher'='RANCHER'
(Other definitions for rancher that I've seen before include "Person with stock" , "US cattle breeder" , "A cowboy" , "Owner of a large farm" , "Stock farmer" .)