Islander starts to regret extra time in prison (6)
I believe the answer is:
cretan
'islander' is the definition.
(I've seen this before)
'starts to regret extra time in prison' is the wordplay.
'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 first letters of 'regret extra time' is 'ret'.
'ret' placed into 'can' is 'CRETAN'.
(Other definitions for cretan that I've seen before include "Native of Greek island south of mainland (6)" , "Island nationality of citizen of Heraklion" , "Of a Greek island, centre of the Minoan civilisation" , "Native of Minoan island" , "A Mediterranean islander" .)