Islander starts to regret extra time in prison (6)
I believe the answer is:
Here is my best explanation:
'islander' is the definition.
'starts to regret extra time in prison' is the subsidiary indication.
'starts to' says to take the initial letters.
'in' means one lot of letters goes inside another.
'prison' becomes 'can' (the can is American slang for prison).
The initial letters of 'regret extra time' is 'ret'.
'ret' going within 'can' is 'cretan'.
(Other definitions for cretan that I've seen before include "Of a Greek island, centre of the Minoan civilisation", "Island nationality of citizen of Heraklion", "Native of Greek island south of mainland (6)", "Native of Minoan island", "Greek islander", "Native of a large Greek island", "A Mediterranean islander", "One from largest Greek isle", "One from Greece's largest island" and "Inhabitant of a large Greek island".)