Convenient pastime as youngster? Not altogether (4)
I believe the answer is:
easy
'convenient' is the definition.
(thesaurus)
'pastime as youngster? not altogether' is the wordplay.
'not altogether' indicates a hidden word.
'EASY' is hidden in the letters of 'pastime as youngster'.
(Other definitions for easy that I've seen before include "Not difficult" , "Relaxed" , "Men may be told to stand thus" , "Frederick Marryat's midshipman" , "... Rider (film)" .)