Extend a period of incarceration (7)
I believe the answer is:
stretch
'extend' is the definition.
(I know that extend can be written as stretch)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stretch that I've seen before include "extension" , "Time lag's got" , "Increase" , "Exercise - period" , "Extend; term in prison" .)