Judge concerned with jailbird pocketing grand (6)
I believe the answer is:
reckon
'judge' is the definition.
(reckoning is a kind of judging)
'concerned with jailbird pocketing grand' is the wordplay.
'concerned with' becomes 're'.
'jailbird' becomes 'con' (I've seen this in other clues).
'pocketing' is an insertion indicator.
'grand' becomes 'k' ('K' can be a synonym of 'grand').
'con' placed around 'k' is 'ckon'.
're'+'ckon'='RECKON'
(Other definitions for reckon that I've seen before include "Believe, suppose, judge" , "Think - calculate" , "Calculate or conclude" , "Count or calculate" , "Consicer or calculate" .)