Fail to turn up for trial, where posh politician's boy is put in prison (4,4)
I believe the answer is:
jump bail
'fail to turn up for trial' is the definition.
(to jump bail is to fail to attend court)
'posh politician's boy is put in prison' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'politician' becomes 'mp' (Member of Parliament).
'boy' becomes 'b' (abbreviation used in schools).
'is put in' indicates putting letters inside.
'prison' becomes 'jail' (I've seen this before).
'u'+'mp'+'b'='umpb'
'umpb' put into 'jail' is 'JUMP BAIL'.
'where' is the link.
(Another definition for jump bail that I've seen is " Abscond from court appearance".)