Not far off, having shaken off an old boy (3)
I believe the answer is:
son
'boy' is the definition.
(I know that boy can be written as son)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for son that I've seen before include "Male child" , "Native" , "Young boy" , "Human offspring" , "Male scion" .)