Trainee pocketing pounds in nightspot (4)
I believe the answer is:
club
'nightspot' is the definition.
(I know that nightspot can be written as club)
'trainee pocketing pounds' is the wordplay.
'trainee' becomes 'cub' (cub can mean a new recruit).
'pocketing' is an insertion indicator (pocket can mean to steal or take possession of).
'pounds' becomes 'l' (L can represent the monetary pound).
'cub' going around 'l' is 'CLUB'.
'in' acts as a link.
(Other definitions for club that I've seen before include "women expect to be in" , "Dance venue" , "Association; cudgel" , "sort of sandwich" , "In which one could be expecting" .)