Bumpkin acceptable in the Old Bull at last (5)
I believe the answer is:
yokel
'bumpkin' is the definition.
(I know that bumpkin can be written as yokel)
'acceptable in the old bull at last' is the wordplay.
'acceptable' becomes 'OK' (OK can mean acceptable or fine).
'in' indicates putting letters inside.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'at last' suggests the final letters.
The last letter of 'bull' is 'l'.
'ok' placed inside 'ye' is 'yoke'.
'yoke'+'l'='YOKEL'
(Other definitions for yokel that I've seen before include "rustic type" , "Bumpkin - rustic" , "Unsophisticated country person" , "Rustic person (derogatory)" , "Simple rustic" .)