In speculation, judge lacking (6)
I believe the answer is:
bereft
'lacking' is the definition.
(thesaurus)
'in speculation judge' is the wordplay.
'in' means one lot of letters goes inside another.
'speculation' becomes 'bet' (bet is a kind of speculation).
'judge' becomes 'ref'.
'bet' going around 'ref' is 'BEREFT'.
(Other definitions for bereft that I've seen before include "Cut off - deprived" , "Deprived, perhaps of reason (6)" , "Sorrowful through loss" , "Deprived of or lacking something" , "Grieving from deprivation" .)