One fine day, will get a spell in jail! (8)
I believe the answer is:
sometime
'one fine day' is the definition.
(thesaurus)
'will get a spell in jail' is the wordplay.
'will' becomes 'some' (I can't justify this - if you can you should believe this answer much more).
'get' is a charade indicator (letters next to each other).
'a spell in jail' becomes 'time' (someone in jail is 'doing time').
'some'+'time'='SOMETIME'
Can you help me to learn more?
(Other definitions for sometime that I've seen before include "Former; occasional (US)" , "At a future unknown period" , "Previous" , "In the future, we don't know when" , "One day" .)