Boy in borstal, formerly (3)
I believe the answer is:
alf
'boy' is the definition.
(I've seen this before)
'in borstal formerly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALF' is hidden within 'borstal formerly'.
(Other definitions for alf that I've seen before include "Chap" , "See man" , "Ramsey starter" , "Great king briefly" , "Boy" .)