Keep new judge in the same cricket side (6)
I believe the answer is:
donjon
'keep' is the definition.
('donjon' can be a synonym of 'keep')
'new judge in the same cricket side' is the wordplay.
'new' becomes 'n'.
'judge' becomes 'j'.
'in the' is an insertion indicator.
'same' becomes 'do'.
'cricket side' becomes 'on'.
'n'+'j'='nj'
'do'+'on'='doon'
'nj' going into 'doon' is 'DONJON'.
(Other definitions for donjon that I've seen before include "Keep of medieval castle" , "tower" , "Castle's central keep" .)