One locked up by judge for unholy crime? (7)
I believe the answer is:
deicide
'unholy crime?' is the definition.
(killing a god)
'one locked up by judge' is the wordplay.
'one' becomes 'i' (Roman numeral).
'locked up by' indicates putting letters inside.
'judge' becomes 'decide' (decide can mean to make a judgment).
'i' placed into 'decide' is 'DEICIDE'.
'for' is the link.
(Other definitions for deicide that I've seen before include "Killing of divine being" , "divine crime" , "Killer" , "The killing of a god" , "Most unholy of sins" .)