Correspondent’s twenty-four hours in federal jail to finish (3,6)
I believe the answer is:
pen friend
'correspondent's' is the definition.
(a pen friend is someone you correspond with)
'twenty-four hours in federal jail to finish' is the wordplay.
'twenty-four hours' becomes 'Fri' (Friday).
'in' indicates putting letters inside.
'federal jail' becomes 'pen' (short for penitentiary - US jail).
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'pen'+'end'='penend'
'fri' inserted into 'penend' is 'PEN-FRIEND'.
(Other definitions for pen friend that I've seen before include "Pal writing" , "Pal to correspond with" , "A pal through correspondence, never met" , "Foreign correspondent" , "Regular correspondent" .)