Delay in sentence supported by prisoner (4,3)
I believe the answer is:
time-lag
'delay' is the definition.
(a time lag is a delay)
'sentence supported by prisoner' is the wordplay.
'sentence' becomes 'time' (a prison sentence).
'supported by' means one lot of letters go next to another (in a down clue, some letters hold up others).
'prisoner' becomes 'lag' (slang term).
'time'+'lag'='TIME-LAG'
'in' acts as a link.
(Other definitions for time-lag that I've seen before include "Period between two connected events" , "Gap between linked events" , "Delay between events" , "Interval between linked events" , "Delay, wait" .)