To work out sentence, convict’s beginning a long stretch? (6)
I believe the answer is:
parsec
'a long stretch?' is the definition.
(astronomical unit of distance)
'to work out sentence convict's beginning' is the wordplay.
'to work out sentence' becomes 'parse'.
'beginning' says to take the initial letters.
The initial letter of 'convicts' is 'c'.
'parse'+'c'='PARSEC'
(Other definitions for parsec that I've seen before include "fair distance" , "Unit for measurement of distance of stars" , "it takes years to get this far" , "Large unit of distance" , "Unit of interstellar distance" .)