Shortly takes an apprentice in a pub (6)
I believe the answer is:
saloon
'a pub' is the definition.
('saloon' can be a synonym of 'pub')
'shortly takes an apprentice in' is the wordplay.
'shortly' becomes 'soon' (synonyms).
'takes' indicates putting letters inside.
'an' becomes 'a'.
'apprentice' becomes 'L' (as in a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'soon' enclosing 'a' is 'saoon'.
'saoon' placed around 'l' is 'SALOON'.
(Other definitions for saloon that I've seen before include "Bar room" , "Pub room" , "Lounge bar" , "US bar - type of car" , "Boozer or shape of car" .)