There's no rush. That will mean prison for years (4,2,4)
I believe the answer is:
lots of time
'there's no rush that will' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'prison for years' is the wordplay.
'prison' becomes 'time' (as in a spell in prison).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'years' becomes 'lotsof' (I can't explain this - if you can you should believe this answer much more).
'time' after 'lotsof' is 'LOTS OF TIME'.
'mean' is the link.
Can you help me to learn more?